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Network

What do you need in order to communicate?

A common language (for the two ends to understand)

A way to address who do you want to communicate with

A Connection (so the content of of the communication can reach the recipients)

What is TCP/IP?

A set of protocols that define how two or more devices can communicate with each other. To learn more about TCP/IP, read here

What is APIPA?

APIPA is a set of it addresses that devices are allocated when the main DHCP server is not reachable

What ip range does APIPA use?

APIPA uses the ip range: 169.254.0.1 - 169.254.255.254.

What is Ethernet?

Ethernet simply refers to the most common type of Local Area Network (LAN) used today. A LAN—in contrast to a WAN (Wide Area Network), which spans a larger geographical area—is a connected network of computers in a small area, like your office, college campus, or even home.

What is a MAC address? What is it used for?

A MAC address is a unique identification number or code used to identify individual devices on the network.

Packets that are sent on the ethernet are always coming from a MAC address and sent to a MAC address. If a network adapter is receiving a packet, it is comparing the packet’s destination MAC address to the adapter’s own MAC address.

When is this MAC address used?: ff:ff:ff:ff:ff:ff

When a device sends a packet to the broadcast MAC address (FF:FF:FF:FF:FF:FF), it is delivered to all stations on the local network. Ethernet broadcasts are used to resolve IP addresses to MAC addresses (by ARP) at the datalink layer .

What is an IP address?

An Internet Protocol address (IP address) is a numerical label assigned to each device connected to a computer network that uses the Internet Protocol for communication.An IP address serves two main functions: host or network interface identification and location addressing.

Explain subnet mask and given an example

A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address. Subnet Mask is made by setting network bits to all "1"s and setting host bits to all "0"s. Within a given network, two host addresses are reserved for special purpose, and cannot be assigned to hosts. The "0" address is assigned a network address and "255" is assigned to a broadcast address, and they cannot be assigned to hosts.

For Example

| Address Class | No of Network Bits | No of Host Bits | Subnet mask | CIDR notation | | ------------- | ------------------ | --------------- | --------------- | ------------- | | A | 8 | 24 | 255.0.0.0 | /8 | | A | 9 | 23 | 255.128.0.0 | /9 | | A | 12 | 20 | 255.240.0.0 | /12 | | A | 14 | 18 | 255.252.0.0 | /14 | | B | 16 | 16 | 255.255.0.0 | /16 | | B | 17 | 15 | 255.255.128.0 | /17 | | B | 20 | 12 | 255.255.240.0 | /20 | | B | 22 | 10 | 255.255.252.0 | /22 | | C | 24 | 8 | 255.255.255.0 | /24 | | C | 25 | 7 | 255.255.255.128 | /25 | | C | 28 | 4 | 255.255.255.240 | /28 | | C | 30 | 2 | 255.255.255.252 | /30 |

What is a private IP address? In which scenarios/system designs, one should use it?

Private IP addresses are assigned to the hosts in the same network to communicate among one another. As the name "private" suggests, the devices having the private IP addresses assigned can't be reached by the devices from any external network. For example, if I am living in a hostel and I want my hostelmates to join the game server I have hosted, I will ask them to join via my server's private IP address, since the network is local to the hostel.

What is a public IP address? In which scenarios/system designs, one should use it?

A public IP address is the public facing IP address. In the event that you was hosting a game server that you want your friends to join, you will give your friends your public IP address to allow their computers to identify and locate your network and server in order for the connection to take place. One time that you would not need to use a public facing IP address is in the event that you was playing with friends who was connected to the same network as you, in that case, you would use a private ip address. In order for someone to be able to connect to your server that is located internally, you will have to setup a port forward to tell your router to allow traffic from the public domain into your network and vice versa.

Explain the OSI model. What layers there are? What each layer is responsible for?

Application: user end (HTTP is here)

Presentation: establishes context between application-layer entities (Encryption is here)

Session: establishes, manages and terminates the connections

Transport: transfers variable-length data sequences from a source to a destination host (TCP & UDP are here)

Network: transfers datagrams from one network to another (IP is here)

Data link: provides a link between two directly connected nodes (MAC is here)

Physical: the electrical and physical spec the data connection (Bits are here)

You can read more about the OSI model in penguintutor.com

For each of the following determine to which OSI layer it belongs:

Error correction
Packets routing
Cables and electrical signals
MAC address
IP address
Terminate connections
3 way handshake

Error correction - Data link

Packets routing - Network

Cables and electrical signals - Physical

MAC address - Data link

IP address - Network

Terminate connections - Session

3 way handshake - Transport

What delivery schemes are you familiar with?

Unitcast: One to one communication where there is one sender and one receiver.

Broadcast: Sending a message to everyone in the network. The address ff:ff:ff:ff:ff:ff is used for broadcasting. Two common protocols which use broadcast are ARP and DHCP.

Multicast: Sending a message to a group of subscribers. It can be one-to-many or many-to-many.

What is CSMA/CD? Is it used in modern ethernet networks?

CSMA/CD stands for Carrier Sense Multiple Access / Collision Detection. Its primarily focus it to manage access to shared medium/bus where only one host can transmit at a given point of time.

CSMA/CD algorithm:

Before sending a frame, it checks whether another host already transmitting a frame.

If no one transmitting, it starts transmitting the frame.

If two hosts transmitted at the same time, we have a collision.

Both hosts stop sending the frame and they send to everyone a 'jam signal' notifying everyone that a collision occurred

They are waiting for a random time before sending again

Once each host waited for a random time, they try to send the frame again and so the

Describe the following network devices and the difference between them:

router
switch
hub

How does a router works?

A router is a physical or virtual appliance that passes information between two or more packet-switched computer networks. A router inspects a given data packet's destination Internet Protocol address (IP address), calculates the best way for it to reach its destination and then forwards it accordingly.

What is NAT?

Network Address Translation (NAT) is a process in which one or more local IP address is translated into one or more Global IP address and vice versa in order to provide Internet access to the local hosts.

What is a proxy? How does it works? What do we need it for?

A proxy server acts as a gateway between you and the internet. It’s an intermediary server separating end users from the websites they browse.

If you’re using a proxy server, internet traffic flows through the proxy server on its way to the address you requested. The request then comes back through that same proxy server (there are exceptions to this rule), and then the proxy server forwards the data received from the website to you.

Proxy servers provide varying levels of functionality, security, and privacy depending on your use case, needs, or company policy.

What is TCP? How does it works? What is the 3 way handshake?

TCP 3-way handshake or three-way handshake is a process which is used in a TCP/IP network to make a connection between server and client.

A three-way handshake is primarily used to create a TCP socket connection. It works when:

A client node sends a SYN data packet over an IP network to a server on the same or an external network. The objective of this packet is to ask/infer if the server is open for new connections.

The target server must have open ports that can accept and initiate new connections. When the server receives the SYN packet from the client node, it responds and returns a confirmation receipt – the ACK packet or SYN/ACK packet.

The client node receives the SYN/ACK from the server and responds with an ACK packet.

What is round-trip delay or round-trip time?

From wikipedia: "the length of time it takes for a signal to be sent plus the length of time it takes for an acknowledgement of that signal to be received"

Bonus question: what is the RTT of LAN?

How does SSL handshake work?

What is the difference between TCP and UDP?

TCP establishes a connection between the client and the server to guarantee the order of the packages, on the other hand, UDP does not establish a connection between client and server and doesn't handle package order. This makes UDP more lightweight than TCP and a perfect candidate for services like streaming.

Penguintutor.com provides a good explanation.

What TCP/IP protocols are you familiar with?

Explain "default gateway"

A default gateway serves as an access point or IP router that a networked computer uses to send information to a computer in another network or the internet.

What is ARP? How does it works?

ARP stands for Address Resolution Protocol. When you try to ping an IP address on your local network, say 192.168.1.1, your system has to turn the IP address 192.168.1.1 into a MAC address. This involves using ARP to resolve the address, hence its name.

Systems keep an ARP look-up table where they store information about what IP addresses are associated with what MAC addresses. When trying to send a packet to an IP address, the system will first consult this table to see if it already knows the MAC address. If there is a value cached, ARP is not used.

What is TTL? What does it helps to prevent?

What is DHCP? How does it works?

It stands for Dynamic Host Configuration Protocol, and allocates IP addresses, subnet masks and gateways to hosts. This is how it works:

A host upon entering a network, broadcasts a message in search of a DHCP server (DHCP DISCOVER)

An offer message is sent back by the DHCP server as a packet containing lease time, subnet mask, IP addresses, etc (DHCP OFFER)

Depending on which offer accepted, the client sends back a reply broadcast letting all DHCP servers know (DHCP REQUEST)

Server sends an acknowledgment (DHCP ACK)

Read more here

Can you have two DHCP servers in the same network? How it works?

What is SSL tunneling? How does it works?

What is a socket? Where can you see the list of sockets in your system?

What is IPv6? Why should we consider using it if we have IPv4?

What is VLAN?

What is MTU?

What happens if you send a packet that is bigger than the MTU?

True or False?. Ping is using UDP because it doesn't care about reliable connection

What is SDN?

What is ICMP? What is it used for?

What is NAT? How does it work?

NAT stands for network address translation. It’s a way to map multiple local private addresses to a public one before transferring the information. Organizations that want multiple devices to employ a single IP address use NAT, as do most home routers. For example, your computer's private IP could be 192.168.1.100, but your router maps the traffic to it's public IP (e.g. 1.1.1.1). Any device on the internet would see the traffic coming from your public IP (1.1.1.1) instead of your private IP (192.168.1.100).

Which factors affect network performances

Which port number is used in each of the following protocols?:

SSH
SMTP
HTTP
DNS
HTTPS
FTP
SFTP

SSH - 22

SMTP - 25

HTTP - 80

DNS - 53

HTTPS - 443

FTP - 21

SFTP - 22

Network - Data and Control planes

What "control plane" refers to?

The control plane is the part of the network that decides how to route and forward packets to a different location.

What "data plane" refers to?

The data plane is the part of the network that actually forwards the data/packets.

What "management plane" refers to?

Refers to monitoring and management functions.

To which plane (data, control, ...) is creating routing tables belongs to?

Control Plane.

Explain Spanning Tree Protocol (STP)

What is link aggregation? Why is it used?

What is Asymmetric Routing? How do deal with it?

What overlay (tunnel) protocols are you familiar with?

What is GRE? How does it works?

What is VXLAN? How does it works?

What is SNAT?

Explain OSPF

What is latency?

Latency is the time taken for an information to reach its destination from the source.

What is bandwidth?

Bandwidth is the capacity of a communication channel to measure how much data the latter can handle over a specific time period. More bandwidth would imply more traffic handling and thus more data transfer.

What is throughput?

Throughput refers to the measurement of the real amount of data transferred over a certain period of time across any transmission channel.

When performing a search query, what is more important, latency or throughput? And how to assure that what managing global infrastructure?

Latency. To have a good latency, a search query should be forwarded to the closest datacenter.

When uploading a video, what is more important, latency or throughput? And how to assure that?

Throughput. To have a good throughput, the upload stream should be routed to an underutilized link.

What other considerations (except latency and throughput) are there when forwarding requests?

Keep caches updated (which means the request could be forwarded not to the closest datacenter)

Explain Spine & Leaf

What is Network Congestion? What can cause it?

What can you tell me about UDP packet format? What about TCP packet format? How is it different?

What is the exponential backoff algorithm? Where is it used?

Using Hamming code, what would be the code word for the following data word 100111010001101?

00110011110100011101

Give examples of protocols found in the application layer

Hypertext Transfer Protocol (HTTP) - used for the webpages on the internet

Simple Mail Transfer Protocol (SMTP) - email transmission

Telecommunications Network - (TELNET) - terminal emulation to allow client access to telnet server

File Transfer Protocol (FTP) - facilitates transfer of files between any two machines

Domain Name System (DNS) - domain name translation

Dynamic Host Configuration Protocol (DHCP) - allocates IP addresses, subnet masks and gateways to hosts

Simple Network Management Protocol (SNMP) - gathers data of devices on the network

Give examples of protocols found in the network Layer

Internet Protocol (IP) - assists in routing packets from one machine to another

Internet Control Message Protocol (ICMP) - lets one know what is going such as error messages and debugging information

What is HSTS?

HTTP Strict Transport Security is a web server directive that informs user agents and web browsers how to handle its connection through a response header sent at the very beginning and back to the browser. This forces connections over HTTPS encryption, disregarding any script's call to load any resource in that domain over HTTP.

Network - Misc

What is the Internet? Is it the same as the World Wide Web?

The internet refers to network of networks, transferring huge amounts of data around the globe.
The World Wide Web is an application running on millions of server, on top of the internet, accessed through what is know as the web browser

What is the ISP?

ISP (Internet Service Provider) is the local internet company provider.

Operating System

Operating System Exercises

Name	Topic	Objective & Instructions	Solution	Comments
Fork 101	Fork	Link	Link
Fork 102	Fork	Link	Link

Operating System - Self Assessment

What is an operating system?

From the book "Operating Systems: Three Easy Pieces":

"responsible for making it easy to run programs (even allowing you to seemingly run many at the same time), allowing programs to share memory, enabling programs to interact with devices, and other fun stuff like that".

Operating System - Process

Can you explain what is a process?

A process is a running program. A program is one or more instructions and the program (or process) is executed by the operating system.

If you had to design an API for processes in an operating system, what would this API look like?

It would support the following:

Create - allow to create new processes

Delete - allow to remove/destroy processes

State - allow to check the state of the process, whether it's running, stopped, waiting, etc.

Stop - allow to stop a running process

How a process is created?

The OS is reading program's code and any additional relevant data

Program's code is loaded into the memory or more specifically, into the address space of the process.

Memory is allocated for program's stack (aka run-time stack). The stack also initialized by the OS with data like argv, argc and parameters to main()

Memory is allocated for program's heap which is required for dynamically allocated data like the data structures linked lists and hash tables

I/O initialization tasks are performed, like in Unix/Linux based systems where each process has 3 file descriptors (input, output and error)

OS is running the program, starting from main()

True or False? The loading of the program into the memory is done eagerly (all at once)

False. It was true in the past but today's operating systems perform lazy loading which means only the relevant pieces required for the process to run are loaded first.

What are different states of a process?

Running - it's executing instructions

Ready - it's ready to run but for different reasons it's on hold

Blocked - it's waiting for some operation to complete. For example I/O disk request

What are some reasons for a process to become blocked?

I/O operations (e.g. Reading from a disk)

Waiting for a packet from a network

What is Inter Process Communication (IPC)?

What is "time sharing"?

Even when using a system with one physical CPU, it's possible to allow multiple users to work on it and run programs. This is possible with time sharing where computing resources are shared in a way it seems to the user the system has multiple CPUs but in fact it's simply one CPU shared by applying multiprogramming and multi-tasking.

What is "space sharing"?

Somewhat the opposite of time sharing. While in time sharing a resource is used for a while by one entity and then the same resource can be used by another resource, in space sharing the space is shared by multiple entities but in a way where it's not being transferred between them.
It's used by one entity until this entity decides to get rid of it. Take for example storage. In storage, a file is yours until you decide to delete it.

What component determines which process runs at a given moment in time?

CPU scheduler

Operating System - Memory

What is "virtual memory" and what purpose it serves?

What is demand paging?

What is copy-on-write or shadowing?

What is a kernel, and what does it do?

The kernel is part of the operating system and is responsible for tasks like:

Allocating memory

Schedule processes

Control CPU

True or False? Some pieces of the code in the kernel are loaded into protected areas of the memory so applications can't overwritten them

True

What is POSIX?

Explain what is Semaphore and what its role in operating systems

What is cache? What is buffer?

Buffer: Reserved place in RAM which is used to hold data for temporary purposes Cache: Cache is usually used when processes reading and writing to the disk to make the process faster by making similar data used by different programs easily accessible.

Virtualization

What is Virtualization?

Virtualization uses software to create an abstraction layer over computer hardware that allows the hardware elements of a single computer—processors, memory, storage and more - to be divided into multiple virtual computers, commonly called virtual machines (VMs).

What is a hypervisor?

Red Hat: "A hypervisor is software that creates and runs virtual machines (VMs). A hypervisor, sometimes called a virtual machine monitor (VMM), isolates the hypervisor operating system and resources from the virtual machines and enables the creation and management of those VMs."

Read more here

What types of hypervisors are there?

Hosted hypervisors and bare-metal hypervisors.

What are the advantages and disadvantges of bare-metal hypervisor over a hosted hypervisor?

Due to having its own drivers and a direct access to hardware components, a baremetal hypervisor will often have better performances along with stability and scalability.

On the other hand, there will probably be some limitation regarding loading (any) drivers so a hosted hypervisor will usually benefit from having a better hardware compatibility.

What types of virtualization are there?

Operating system virtualization Network functions virtualization Desktop virtualization

Is containerization is a type of Virtualization?

Yes, it's a operating-system-level virtualization, where the kernel is shared and allows to use multiple isolated user-spaces instances.

How the introduction of virtual machines changed the industry and the way applications were deployed?

The introduction of virtual machines allowed companies to deploy multiple business applications on the same hardware while each application is separated from each other in secured way, where each is running on its own separate operating system.

Virtual Machines

Do we need virtual machines in the age of containers? Are they still relevant?

Python - OOP

Explain inheritance and how to use it in Python

Explain and demonstrate class attributes & instance attributes

In the following block of code x is a class attribute while self.y is a instance attribute

class MyClass(object): x = 1 def __init__(self, y): self.y = y

Python - Exceptions

What is an error? What is an exception? What types of exceptions are you familiar with?

# Note that you generally don't need to know the compiling process but knowing where everything comes from # and giving complete answers shows that you truly know what you are talking about. Generally, every compiling process have a two steps. - Analysis - Code Generation. Analysis can be broken into: 1. Lexical analysis (Tokenizes source code) 2. Syntactic analysis (Check whether the tokens are legal or not, tldr, if syntax is correct) for i in 'foo' ^ SyntaxError: invalid syntax We missed ':' 3. Semantic analysis (Contextual analysis, legal syntax can still trigger errors, did you try to divide by 0, hash a mutable object or use an undeclared function?) 1/0 ZeroDivisionError: division by zero These three analysis steps are the responsible for error handlings. The second step would be responsible for errors, mostly syntax errors, the most common error. The third step would be responsible for Exceptions. As we have seen, Exceptions are semantic errors, there are many builtin Exceptions: ImportError ValueError KeyError FileNotFoundError IndentationError IndexError ... You can also have user defined Exceptions that have to inherit from the `Exception` class, directly or indirectly. Basic example: class DividedBy2Error(Exception): def __init__(self, message): self.message = message def division(dividend,divisor): if divisor == 2: raise DividedBy2Error('I dont want you to divide by 2!') return dividend / divisor division(100, 2) >>> __main__.DividedBy2Error: I dont want you to divide by 2!

Explain Exception Handling and how to use it in Python

Exceptions: Errors detected during execution are called Exceptions.

Handling Exception: When an error occurs, or exception as we call it, Python will normally stop and generate an error message.
Exceptions can be handled using try and except statement in python.

Example: Following example asks the user for input until a valid integer has been entered.
If user enter a non-integer value it will raise exception and using except it will catch that exception and ask the user to enter valid integer again.

while True: try: a = int(input("please enter an integer value: ")) break except ValueError: print("Ops! Please enter a valid integer value.")

For more details about errors and exceptions follow this https://docs.python.org/3/tutorial/errors.html

What is the result of running the following function?

def true_or_false():
    try:
        return True
    finally:
        return False

False

Python Built-in functions

Explain the following built-in functions (their purpose + use case example):

repr
any
all

What is the difference between repr function and str?

What is the __call__ method?

It is used to emulate callable objects. It allows a class instance to be called as a function.

Example code:

class Foo: def __init__(self: object) -> None: pass def __call__(self: object) -> None: print("Called!") f = Foo() f()

Result:

Called!

Do classes has the __call__ method as well? What for?

What _ is used for in Python?

Translation lookup in i18n

Hold the result of the last executed expression or statement in the interactive interpreter.

As a general purpose "throwaway" variable name. For example: x, y, _ = get_data() (x and y are used but since we don't care about third variable, we "threw it away").

Explain what is GIL

Python Global Interpreter Lock (GIL) is a type of process lock which is used by python whenever it deals with processes. Generally, Python only uses only one thread to execute the set of written statements. This means that in python only one thread will be executed at a time

What is Lambda? How is it used?

A lambda expression is an 'anonymous' function, the difference from a normal defined function using the keyword `def`` is the syntax and usage.

The syntax is:

lambda[parameters]: [expresion]

Examples:

A lambda function add 10 with any argument passed.

x = lambda a: a + 10 print(x(10))

An addition function

addition = lambda x, y: x + y print(addition(10, 20))

Squaring function

square = lambda x : x ** 2 print(square(5))

Generally it is considered a bad practice under PEP 8 to assign a lambda expresion, they are meant to be used as parameters and inside of other defined functions.

Properties

Are there private variables in Python? How would you make an attribute of a class, private?

Explain the following:

getter
setter
deleter

Explain what is @property

How do you swap values between two variables?

x, y = y, x

Explain the following object's magic variables:

dict

Write a function to return the sum of one or more numbers. The user will decide how many numbers to use

First you ask the user for the amount of numbers that will be use. Use a while loop that runs until amount_of_numbers becomes 0 through subtracting amount_of_numbers by one each loop. In the while loop you want ask the user for a number which will be added a variable each time the loop runs.

def return_sum(): amount_of_numbers = int(input("How many numbers? ")) total_sum = 0 while amount_of_numbers != 0: num = int(input("Input a number. ")) total_sum += num amount_of_numbers -= 1 return total_sum

Print the average of [2, 5, 6]. It should be rounded to 3 decimal places

li = [2, 5, 6] print("{0:.3f}".format(sum(li)/len(li)))

Python - Lists

What is a tuple in Python? What is it used for?

A tuple is a built-in data type in Python. It's used for storing multiple items in a single variable.

List, like a tuple, is also used for storing multiple items. What is then, the difference between a tuple and a list?

List, as opposed to a tuple, is a mutable data type. It means we can modify it and at items to it.

How to add the number 2 to the list x = [1, 2, 3]

x.append(2)

How to get the last element of a list?

some_list[-1]

How to add the items of [1, 2, 3] to the list [4, 5, 6]?

x = [4, 5, 6] x.extend([1, 2, 3])

Don't use append unless you would like the list as one item.

How to remove the first 3 items from a list?

my_list[0:3] = []

How to insert an item to the beginning of a list? What about two items?

One item:

numbers = [1, 2, 3, 4, 5] numbers.insert(0, 0) print(numbers)

Multiple items or list:

numbers_1 = [2, 3, 4, 5] numbers_2 = [0, 1] numbers_1 = numbers_2 + numbers_1 print(numbers_1)

How to sort list by the length of items?

sorted_li = sorted(li, key=len)

Or without creating a new list:

li.sort(key=len)

Do you know what is the difference between list.sort() and sorted(list)?

sorted(list) will return a new list (original list doesn't change)

list.sort() will return None but the list is change in-place

sorted() works on any iterable (Dictionaries, Strings, ...)

list.sort() is faster than sorted(list) in case of Lists

Convert every string to an integer: [['1', '2', '3'], ['4', '5', '6']]

nested_li = [['1', '2', '3'], ['4', '5', '6']] [[int(x) for x in li] for li in nested_li]

How to merge two sorted lists into one sorted list?

sorted(li1 + li2)

Another way:

i, j = 0 merged_li = [] while i < len(li1) and j < len(li2): if li1[i] < li2[j]: merged_li.append(li1[i]) i += 1 else: merged_li.append(li2[j]) j += 1 merged_li = merged_li + merged_li[i:] + merged_li[j:]

How to check if all the elements in a given lists are unique? so [1, 2, 3] is unique but [1, 1, 2, 3] is not unique because 1 exists twice

There are many ways of solving this problem:
# Note: :list and -> bool are just python typings, they are not needed for the correct execution of the algorithm.

Taking advantage of sets and len:

def is_unique(l:list) -> bool:
    return len(set(l)) == len(l)

This one is can be seen used in other programming languages.

def is_unique2(l:list) -> bool:
    seen = []

    for i in l:
        if i in seen:
            return False
        seen.append(i)
    return True

Here we just count and make sure every element is just repeated once.

def is_unique3(l:list) -> bool:
    for i in l:
        if l.count(i) > 1:
            return False
    return True

This one might look more convulated but hey, one liners.

def is_unique4(l:list) -> bool:
    return all(map(lambda x: l.count(x) < 2, l))

You have the following function

def my_func(li = []):
    li.append("hmm")
    print(li)

If we call it 3 times, what would be the result each call?

['hmm'] ['hmm', 'hmm'] ['hmm', 'hmm', 'hmm']

How to iterate over a list?

for item in some_list: print(item)

How to iterate over a list with indexes?

for i, item in enumerate(some_list): print(i)

How to start list iteration from 2nd index?

Using range like this

for i in range(1, len(some_list)): some_list[i]

Another way is using slicing

for i in some_list[1:]:

How to iterate over a list in reverse order?

Method 1

for i in reversed(li): ...

Method 2

n = len(li) - 1 while n > 0: ... n -= 1

Sort a list of lists by the second item of each nested list

li = [[1, 4], [2, 1], [3, 9], [4, 2], [4, 5]] sorted(li, key=lambda l: l[1])

or

li.sort(key=lambda l: l[1)

Combine [1, 2, 3] and ['x', 'y', 'z'] so the result is [(1, 'x'), (2, 'y'), (3, 'z')]

nums = [1, 2, 3] letters = ['x', 'y', 'z'] list(zip(nums, letters))

What is List Comprehension? Is it better than a typical loop? Why? Can you demonstrate how to use it?

From Docs: "List comprehensions provide a concise way to create lists. Common applications are to make new lists where each element is the result of some operations applied to each member of another sequence or iterable, or to create a subsequence of those elements that satisfy a certain condition.".

It's better because they're compact, faster and have better readability.

For loop:

number_lists = [[1, 7, 3, 1], [13, 93, 23, 12], [123, 423, 456, 653, 124]] odd_numbers = [] for number_list in number_lists: for number in number_list: if number % 2 == 0: odd_numbers.append(number) print(odd_numbers)

List comprehesion:

number_lists = [[1, 7, 3, 1], [13, 93, 23, 12], [123, 423, 456, 653, 124]] odd_numbers = [number for number_list in number_lists for number in number_list if number % 2 == 0] print(odd_numbers)

You have the following list: [{'name': 'Mario', 'food': ['mushrooms', 'goombas']}, {'name': 'Luigi', 'food': ['mushrooms', 'turtles']}] Extract all type of foods. Final output should be: {'mushrooms', 'goombas', 'turtles'}

brothers_menu = \ [{'name': 'Mario', 'food': ['mushrooms', 'goombas']}, {'name': 'Luigi', 'food': ['mushrooms', 'turtles']}] # "Classic" Way def get_food(brothers_menu) -> set: temp = [] for brother in brothers_menu: for food in brother['food']: temp.append(food) return set(temp) # One liner way (Using list comprehension) set([food for bro in x for food in bro['food']])

Python - Dictionaries

How to create a dictionary?

my_dict = dict(x=1, y=2) OR my_dict = {'x': 1, 'y': 2} OR my_dict = dict([('x', 1), ('y', 2)])

How to remove a key from a dictionary?

del my_dict['some_key'] you can also use my_dict.pop('some_key') which returns the value of the key.

How to sort a dictionary by values?

{k: v for k, v in sorted(x.items(), key=lambda item: item[1])}

How to sort a dictionary by keys?

dict(sorted(some_dictionary.items()))

How to merge two dictionaries?

some_dict1.update(some_dict2)

Convert the string "a.b.c" to the dictionary {'a': {'b': {'c': 1}}}

output = {} string = "a.b.c" path = string.split('.') target = reduce(lambda d, k: d.setdefault(k, {}), path[:-1], output) target[path[-1]] = 1 print(output)

Common Algorithms Implementation

Can you implement "binary search" in Python?

Solution

Python Files

How to write to a file?

with open('file.txt', 'w') as file: file.write("My insightful comment")

Sum all the integers in a given file

Print a random line of a given file

Print every 3rd line of a given file

Print the number of lines in a given file

Print the number of of words in a given file

Can you write a function which will print all the file in a given directory? including sub-directories

Write a dictionary (variable) to a file

import json with open('file.json', 'w') as f: f.write(json.dumps(dict_var))

Python OS

How to print current working directory?

import os print(os.getcwd())

Given the path /dir1/dir2/file1 print the file name (file1)

import os print(os.path.basename('/dir1/dir2/file1')) # Another way print(os.path.split('/dir1/dir2/file1')[1])

Given the path /dir1/dir2/file1

Print the path without the file name (/dir1/dir2)
Print the name of the directory where the file resides (dir2)

import os ## Part 1. # os.path.dirname gives path removing the end component dirpath = os.path.dirname('/dir1/dir2/file1') print(dirpath) ## Part 2. print(os.path.basename(dirpath))

How do you execute shell commands using Python?

How do you join path components? for example /home and luig will result in /home/luigi

How do you remove non-empty directory?

Python Regex

How do you perform regular expressions related operations in Python? (match patterns, substitute strings, etc.)

Using the re module

How to find all the IP addresses in a variable? How to find them in a file?

Python Strings

Find the first repeated character in a string

While you iterate through the characters, store them in a dictionary and check for every character whether it's already in the dictionary.

def firstRepeatedCharacter(str): chars = {} for ch in str: if ch in chars: return ch else: chars[ch] = 0

How to extract the unique characters from a string? for example given the input "itssssssameeeemarioooooo" the output will be "mrtisaoe"

x = "itssssssameeeemarioooooo" y = ''.join(set(x))

Find all the permutations of a given string

def permute_string(string): if len(string) == 1: return [string] permutations = [] for i in range(len(string)): swaps = permute_string(string[:i] + string[(i+1):]) for swap in swaps: permutations.append(string[i] + swap) return permutations print(permute_string("abc"))

Short way (but probably not acceptable in interviews):

from itertools import permutations [''.join(p) for p in permutations("abc")]

Detailed answer can be found here: http://codingshell.com/python-all-string-permutations

How to check if a string contains a sub string?

Find the frequency of each character in string

Count the number of spaces in a string

You can use the "count" method like this:

ImAString.count(" ")

Given a string, find the N most repeated words

Given the string (which represents a matrix) "1 2 3\n4 5 6\n7 8 9" create rows and colums variables (should contain integers, not strings)

What is the result of each of the following?

>> ', '.join(["One", "Two", "Three"])
>> " ".join("welladsadgadoneadsadga".split("adsadga")[:2])
>> "".join(["c", "t", "o", "a", "o", "q", "l"])[0::2]

>>> 'One, Two, Three' >>> 'well done' >>> 'cool'

If x = "pizza", what would be the result of x[::-1]?

It will reverse the string, so x would be equal to azzip.

Reverse each word in a string (while keeping the order)

What is the output of the following code: "".join(["a", "h", "m", "a", "h", "a", "n", "q", "r", "l", "o", "i", "f", "o", "o"])[2::3]

mario

Python Iterators

What is an iterator?

Python Misc

Explain data serialization and how do you perform it with Python

How do you handle argument parsing in Python?

What is a generator? Why using generators?

What would be the output of the following block?

for i in range(3, 3):
   print(i)

No output :)

What is yeild? When would you use it?

Explain the following types of methods and how to use them:

Static method
Class method
instance method

How to reverse a list?

How to combine list of strings into one string with spaces between the strings

You have the following list of nested lists: [['Mario', 90], ['Geralt', 82], ['Gordon', 88]] How to sort the list by the numbers in the nested lists?

One way is:

the_list.sort(key=lambda x: x[1])

Explain the following:

zip()
map()
filter()

Python - Slicing

For the following slicing exercises, assume you have the following list: my_list = [8, 2, 1, 10, 5, 4, 3, 9]

What is the result of `my_list[0:4]`?

What is the result of `my_list[5:6]`?

What is the result of `my_list[5:5]`?

What is the result of `my_list[::-1]`?

What is the result of `my_list[::3]`?

What is the result of `my_list[2:]`?

What is the result of `my_list[:3]`?

Python Debugging

How do you debug Python code?

pdb :D

How to check how much time it took to execute a certain script or block of code?

What empty return returns?

Short answer is: It returns a None object.

We could go a bit deeper and explain the difference between

def a ():
    return

>>> None

And

def a ():
    pass

>>> None

Or we could be asked this as a following question, since they both give the same result.

We could use the dis module to see what's going on:

  2           0 LOAD_CONST               0 (<code object a at 0x0000029C4D3C2DB0, file "<dis>", line 2>)
              2 LOAD_CONST               1 ('a')
              4 MAKE_FUNCTION            0
              6 STORE_NAME               0 (a)

  5           8 LOAD_CONST               2 (<code object b at 0x0000029C4D3C2ED0, file "<dis>", line 5>)
             10 LOAD_CONST               3 ('b')
             12 MAKE_FUNCTION            0
             14 STORE_NAME               1 (b)
             16 LOAD_CONST               4 (None)
             18 RETURN_VALUE

Disassembly of <code object a at 0x0000029C4D3C2DB0, file "<dis>", line 2>:
  3           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

Disassembly of <code object b at 0x0000029C4D3C2ED0, file "<dis>", line 5>:
  6           0 LOAD_CONST               0 (None)
              2 RETURN_VALUE

An empty return is exactly the same as return None and functions without any explicit return will always return None regardless of the operations, therefore

def sum(a, b):
    global c
    c = a + b

>>> None

How to improve the following block of code?

li = []
for i in range(1, 10):
    li.append(i)

[i for i in range(1, 10)]

Given the following function

def is_int(num):
    if isinstance(num, int):
        print('Yes')
    else:
        print('No')

What would be the result of is_int(2) and is_int(False)?

Python - Linked List

Can you implement a linked list in Python?

The reason we need to implement in the first place, it's because a linked list isn't part of Python standard library.
To implement a linked list, we have to implement two structures: The linked list itself and a node which is used by the linked list.

Let's start with a node. A node has some value (the data it holds) and a pointer to the next node

class Node(object): def __init__(self, data): self.data = data self.next = None

Now the linked list. An empty linked list has nothing but an empty head.

class LinkedList(object): def __init__(self): self.head = None

Now we can start using the linked list

ll = Linkedlist() ll.head = Node(1) ll.head.next = Node(2) ll.head.next.next = Node(3)

What we have is:

| 1 | -> | 2 | -> | 3 |

Usually, more methods are implemented, like a push_head() method where you insert a node at the beginning of the linked list

def push_head(self, value): new_node = Node(value) new_node.next = self.head self.head = new_node

Add a method to the Linked List class to traverse (print every node's data) the linked list

def print_list(self): node = self.head while(node): print(node.data) node = node.next

Write a method to that will return a boolean based on whether there is a loop in a linked list or not

Let's use the Floyd's Cycle-Finding algorithm:

def loop_exists(self): one_step_p = self.head two_steps_p = self.head while(one_step_p and two_steps_p and two_steps_p.next): one_step_p = self.head.next two_step_p = self.head.next.next if (one_step_p == two_steps_p): return True return False

Python - Stack

Implement Stack in Python

Python Testing

What is your experience with writing tests in Python?

What is PEP8? Give an example of 3 style guidelines

PEP8 is a list of coding conventions and style guidelines for Python

5 style guidelines:

1. Limit all lines to a maximum of 79 characters. 2. Surround top-level function and class definitions with two blank lines. 3. Use commas when making a tuple of one element 4. Use spaces (and not tabs) for indentation 5. Use 4 spaces per indentation level

How to test if an exception was raised?

What assert does in Python?

Explain mocks

How do you measure execution time of small code snippets?

Why one shouldn't use assert in non-test/production code?

Flask

Can you describe what is Django/Flask and how you have used it? Why Flask and not Django? (or vice versa)

What is a route?

As every web framework, Flask provides a route functionality that lets you serve a content of a given URL.
There are multiple ways to map a URL with a function in Python.

Decorator: you can use python decorators. In this case we're using app. This app decorator is the instance of the Flask class. And route it's a method of this class.

@app.route('/') def home(): return 'main website'

add_url_rule method: This is a method of the Flask class. We can also use it for map the URL with a function.

def home(): return 'main website' app.add_url_rule('/', view_func=home)

What is a blueprint in Flask?

What is a template?

zip

Given x = [1, 2, 3], what is the result of list(zip(x))?

[(1,), (2,), (3,)]

What is the result of each of the following:

list(zip(range(5), range(50), range(50)))
list(zip(range(5), range(50), range(-2)))

[(0, 0, 0), (1, 1, 1), (2, 2, 2), (3, 3, 3), (4, 4, 4)] []

Python Descriptors

Explain Descriptors

Read about descriptors here

What would be the result of running a.num2 assuming the following code

class B:
    def __get__(self, obj, objtype=None):
        reuturn 10

class A:
    num1 = 2
    num2 = Five()

10

What would be the result of running some_car = Car("Red", 4) assuming the following code

class Print:

    def __get__(self, obj, objtype=None):
        value = obj._color
        print("Color was set to {}".format(valie))
        return value

    def __set__(self, obj, value):
        print("The color of the car is {}".format(value))
        obj._color = value

class Car:

    color = Print()

    def __ini__(self, color, age):
        self.color = color
        self.age = age

An instance of Car class will be created and the following will be printed: "The color of the car is Red"

Python Misc

How can you spawn multiple processes with Python?

Implement simple calculator for two numbers

def add(num1, num2): return num1 + num2 def sub(num1, num2): return num1 - num2 def mul(num1, num2): return num1*num2 def div(num1, num2): return num1 / num2 operators = { '+': add, '-': sub, '*': mul, '/': div } if __name__ == '__main__': operator = str(input("Operator: ")) num1 = int(input("1st number: ")) num2 = int(input("2nd number: ")) print(operators[operator](num1, num2))

What data types are you familiar with that are not Python built-in types but still provided by modules which are part of the standard library?

This is a good reference https://docs.python.org/3/library/datatypes.html

Explain what is a decorator

In python, everything is an object, even functions themselves. Therefore you could pass functions as arguments for another function eg;
def wee(word): return word def oh(f): return f + "Ohh" >>> oh(wee("Wee")) <<< Wee Ohh

This allows us to control the before execution of any given function and if we added another function as wrapper, (a function receiving another function that receives a function as parameter) we could also control the after execution.

Sometimes we want to control the before-after execution of many functions and it would get tedious to write

f = function(function_1()) f = function(function_1(function_2(*args)))

every time, that's what decorators do, they introduce syntax to write all of this on the go, using the keyword '@'.

Can you show how to write and use decorators?

These two decorators (ntimes and timer) are usually used to display decorators functionalities, you can find them in lots of tutorials/reviews. I first saw these examples two years ago in pyData 2017. https://www.youtube.com/watch?v=7lmCu8wz8ro&t=3731s
Simple decorator: def deco(f): print(f"Hi I am the {f.__name__}() function!") return f @deco def hello_world(): return "Hi, I'm in!" a = hello_world() print(a) >>> Hi I am the hello_world() function! Hi, I'm in!

This is the simplest decorator version, it basically saves us from writting a = deco(hello_world()). But at this point we can only control the before execution, let's take on the after:

def deco(f): def wrapper(*args, **kwargs): print("Rick Sanchez!") func = f(*args, **kwargs) print("I'm in!") return func return wrapper @deco def f(word): print(word) a = f("************") >>> Rick Sanchez! ************ I'm in!

deco receives a function -> f wrapper receives the arguments -> *args, **kwargs

wrapper returns the function plus the arguments -> f(*args, **kwargs) deco returns wrapper.

As you can see we conveniently do things before and after the execution of a given function.

For example, we could write a decorator that calculates the execution time of a function.

import time def deco(f): def wrapper(*args, **kwargs): before = time.time() func = f(*args, **kwargs) after = time.time() print(after-before) return func return wrapper @deco def f(): time.sleep(2) print("************") a = f() >>> 2.0008859634399414

Or create a decorator that executes a function n times.

def n_times(n): def wrapper(f): def inner(*args, **kwargs): for _ in range(n): func = f(*args, **kwargs) return func return inner return wrapper @n_times(4) def f(): print("************") a = f() >>>************ ************ ************ ************

Write a decorator that calculates the execution time of a function

Write a script which will determine if a given host is accessible on a given port

Are you familiar with Dataclasses? Can you explain what are they used for?

You wrote a class to represent a car. How would you compare two cars instances if two cars are equal if they have the same model and color?

class Car: def __init__(self, model, color): self.model = model self.color = color def __eq__(self, other): if not isinstance(other, Car): return NotImplemented return self.model == other.model and self.color == other.color >> a = Car('model_1', 'red') >> b = Car('model_2', 'green') >> c = Car('model_1', 'red') >> a == b False >> a == c True

Explain Context Manager

Tell me everything you know about concurrency in Python

Explain the Buffer Protocol

Do you have experience with web scraping? Can you describe what have you used and for what?

Can you implement Linux's tail command in Python? Bonus: implement head as well

You have created a web page where a user can upload a document. But the function which reads the uploaded files, runs for a long time, based on the document size and user has to wait for the read operation to complete before he/she can continue using the web site. How can you overcome this?

How yield works exactly?

Monitoring

Explain monitoring. What is it? What its goal?

Google: "Monitoring is one of the primary means by which service owners keep track of a system’s health and availability".

What is wrong with the old approach of watching for a specific value and trigger an email/phone alert while value is exceeded?

This approach require from a human to always check why the value exceeded and how to handle it while today, it is more effective to notify people only when they need to take an actual action. If the issue doesn't require any human intervention, then the problem can be fixed by some processes running in the relevant environment.

What types of monitoring outputs are you familiar with and/or used in the past?

Alerts
Tickets
Logging

What is the difference between infrastructure monitoring and application monitoring? (methods, tools, ...)

Prometheus

What is Prometheus? What are some of Prometheus's main features?

In what scenarios it might be better to NOT use Prometheus?

From Prometheus documentation: "if you need 100% accuracy, such as for per-request billing".

Describe Prometheus architecture and components

Can you compare Prometheus to other solutions like InfluxDB for example?

What is an Alert?

Describe the following Prometheus components:

Prometheus server
Push Gateway
Alert Manager

Prometheus server is responsible for scraping and storing the data
Push gateway is used for short-lived jobs
Alert manager is responsible for alerts ;)

What is an Instance? What is a Job?

What core metrics types Prometheus supports?

What is an exporter? What is it used for?

Which Prometheus best practices are you familiar with?. Name at least three

How to get total requests in a given period of time?

What HA in Prometheus means?

How do you join two metrics?

How to write a query that returns the value of a label?

How do you convert cpu_user_seconds to cpu usage in percentage?

Go

What are some characteristics of the Go programming language?

Strong and static typing - the type of the variables can't be changed over time and they have to be defined at compile time

Simplicity

Fast compile times

Built-in concurrency

Garbage collected

Platform independent

Compile to standalone binary - anything you need to run your app will be compiled into one binary. Very useful for version management in run-time.

Go also has good community.

What is the difference between var x int = 2 and x := 2?

The result is the same, a variable with the value 2.

With var x int = 2 we are setting the variable type to integer while with x := 2 we are letting Go figure out by itself the type.

True or False? In Go we can redeclare variables and once declared we must use it.

False. We can't redeclare variables but yes, we must used declared variables.

What libraries of Go have you used?

This should be answered based on your usage but some examples are:

fmt - formatted I/O

What is the problem with the following block of code? How to fix it?

func main() {
    var x float32 = 13.5
    var y int
    y = x
}

The following block of code tries to convert the integer 101 to a string but instead we get "e". Why is that? How to fix it?

package main

import "fmt"

func main() {
    var x int = 101
    var y string
    y = string(x)
    fmt.Println(y)
}

It looks what unicode value is set at 101 and uses it for converting the integer to a string. If you want to get "101" you should use the package "strconv" and replace y = string(x) with y = strconv.Itoa(x)

What is wrong with the following code?:

package main

func main() {
    var x = 2
    var y = 3
    const someConst = x + y
}

Constants in Go can only be declared using constant expressions. But x, y and their sum is variable.
const initializer x + y is not a constant