Python: Getting Started

Date: May 2019 - June 2019

Python 2 vs Python 3

TLDR: Just use Python 3.

Python 3 adoption did not go smoothly. Today (May 2019) you will still see Python 2 being used, mostly 2.7 versions.

Python 2	Python 3
Last version 2.7.13 released in Dec. 2016	Released Dec. 2008
Maintained, but no new features	New features being added
End of Life (EOL) in 2020	Unicode supported by default
Still default on many systems (ie: Ubuntu 18.04)	Cleared some Python 2 confusion
Syntax: `print "Hello World"`	Syntax: `print("Hello World")`
	Minor differences from Python 2

Not a lot of differences in terms of syntax.

Types, Statements, Loops, Exceptions

Dynamically typed
Types are inferred by the compiler
Type hinting
- New feature added in Python 3.5
- Allows you to annotate types

def add_numbers(a: int, b: int) -> int:
    return a + b

Only used to inform editors and IDEs
- Does not prevent code from running if incorrect types are used
Important to write a lot of unit tests for Python scripts

Integers and Floats

Integers are numbers. Floats are decimal numbers.

Defining an integrer:

answer = 42

Defining a float:

pi = 3.13159

Python 3 also introduces complex numbers as a type.

Python will not throw a type error when doing operations between number types:

answer + pi # 45.14159

Casting a value to a type:

int(pi) == 3
float(answer) == 42.0

Strings

Python 3 defaults strings to Unicode text. Python 2 defaults to ASCII.

Strings can be defined with single quotes ', double quotes ", or three times quotes (single or double) ''' """:

'Hello World' == "Hello World" == """Hello World"""

Three times quotes are often used as function or class documentation. While # are used for comments in Python there are no operators for multi-line comments. Three times quotes are used for multi-line strings: it is acceptable to use them as multi-line comments that are not assigned to any variables. Your editor may even recognize it as a comment.

Useful string methods in python:

"hello".capitalize() == "Hello"
"hello".replace("e", "a") == "hallo"
"hello".isalpha() == True
"123".isdigit() == True # Useful when converting to int
"some,csv,values".split(",") == ["some", "csv", "values"]

String Format Functions

String format functions are used to interpolate values into strings:

name = "World"
machine = "HAL"
"Nice to meet you {0}. I am {1}".format(name, machine)

Format String Literals

Python 3.6 introduces string interpolation, known as format string literals:

f"Nice to meet you {name}. I am {machine}"

Boolean and None

Variables can be declared as boolean by assigning them as True or False:

python_course = True
java_course = False

Booleans can be converted to integer or string:

int(python_course) == 1
int(java_course) == 0
str(python_course) == "True"

None is similar to null in other languages:

aliens_found = None

None is useful as a placeholder value, for variables that are defined in advance but not yet assigned a value. None evaluates to False in conditional statements.

None's type is called NoneType:

type(None) # <class 'NoneType'>

If Statements

Normal if statement in python:

number = 5
if number == 5:
    print("Number is 5")
else:
    print("Number is NOT 5")

if and else statements both end with a colon :. This is a requirement in python.

We use the == to check for equality. We can also use the operator is to see if two objects are pointing to the same value in memory.

Python can also evaluate a value for truthiness and falsiness:

Any number other than 0 has a truthy value
Any non-empty string has a truthy value
Any non-empty list has a truthy value
None has a falsey value

!= is the conditional operator to check for lack of equality. not is the negation operator and is not can be used to check the negative case of the is operator:

if not python_course:
    print("This statement will NOT evaluate")

and and or operators in python can be used to check multiple conditions:

number = 3
python_course = True

if number == 3 and python_course:
    print("This will evaluate")

if number == 17 or python_course:
    print("This will also evaluate")

Ternary If Statements

Python does not use the ? operator like other languages for ternary statements:

a = 1
b = 2
print("bigger" if a > b else "smaller") # "smaller"

Lists

To define a list in python, use the square brackets:

student_names = [] # empty list
student_names = ["Mark", "Katarina", "Jessica"] # populated list

Lists are 0-indexed and can be accessed by index:

student_names[0] == "Mark"
student_names[2] == "Jessica"

Accessing elements from the end of the list is done with negative indicies. To access the last element in a list use an index of -1:

student_names[-1] == "Jessica"
student_names[-3] == "Mark"

To add elements to the end of a list, use the append method:

student_names.append("Homer")
student_names == ["Mark", "Katarina", "Jessica", "Homer"]

To check for the existence of an element in a list use the in operator:

"Mark" in student_names == True

To check the length of a list use the python function len():

len(student_list) == 4

Lists in python are similar to arrays in other languages, but with added benefits: having multiple types in a single list is allowed. As a best practice it may be a good idea to restrict a list to single type - it could help avoid potential bugs.

To remove an element from a list, use the del keyword:

del student_names[2]
student_names == ["Mark, "Katarina", "Homer"]

List Slicing

Slicing lists will not mutate the list. The operation is done with the : in the index of a list:

student_names = ["Mark", "Katarina", "Homer"]
student_names[1:] == ["Katarina", "Homer"]
student_names[:-1] == ["Mark", "Katarina"]
student_names[1:-1] == ["Katarina"]

Loops

Syntax for a for loop:

for name in student_names:
    print("Student name is {0}".format(name))

Range

You can also use the range function in python:

x = 0
for index in range(10):
    x += 10
    print("The value of x is {0}".format(x))

range() takes a value and creates a list whose elements begin at 0 and end at value - 1:

list(range(10)) == [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

range() also supports two arguments, a starting value and a value to end before:

list(range(5, 10)) == [5, 6, 7, 8, 9]

range() also supports three arguments, with the last value determines the increments:

list(range(2, 10, 2)) == [2, 4, 6, 8]

Break and Continue

break will cause your loop to stop executing and exit without reaching the end of the list or range function:

student_names = ["James", "Katarina", "Jessica", "Mark", "Bort", "Frank Grimes", "Max Power"]

for name in student_names:
    if name == "Mark":
        print("Found him! {0}".format(name))
        break
    print("Currently testing {0}".format(name))

continue tells a loop to exit the current iteration and continue to the next:

for name in student_names:
    if name == "Bort":
        continue
    print("Currently testing {0}".format(name))

While Loops

while loops can also make use of break and continue. while loop syntax:

x = 0
while x < 10:
    print("Count is {0}".format(x))
    x += 1

while loops check the condition before even entering the loop.

Dictionaries

Allows the storage of key/value pairs. Very similar to JSON (hash?).

Syntax for defining a dictionary:

student = {
    "id": 15163,
    "name": "Mark",
    "feedback": None
}

Dictionaries are very valuable when you need to store structured data. You can group several dictionaries together in a list:

all_students = [
    { "id": 15163, "name": "Mark" }
    { "id": 63112, "name": "Katarina" }
    { "id": 30021, "name": "Jessica" }
]

Syntax for reading a dictionary value:

student["name"] == "Mark"

Python will raise an exception for non-existent keys:

student["last_name"] == KeyError

You can set default values for keys to avoid key errors:

student.get("last_name", "Unknown") == "Unknown"
student["last_name"] == "Unknown"

Retrieve a list of keys or values of a dictionary:

student.keys() == ["id", "name", "feedback"]
student.values() == [15163, "Mark", None]

Removing a key is the same as removing an element in a list, using the del operator:

del student["name"]
student == { "id": 15163, "feedback": None }

Exceptions

Exceptions are events that occur during your program's excution that cause your program to stop executing. It generally means that some error has occurred and your program does not know how to deal with it.

There are ways to handle exceptions:

student = {
    "id": 15163,
    "name": "Mark",
    "feedback": None
}

try:
    last_name = student["last_name"]
except KeyError:
    print("Error finding last_name")

print("The program was not halted by the exeception")

Catch multiple exceptions using multiple except blocks:

try:
    last_name = student["last_name"]
    numbered_last_name = 3 + last_name
except KeyError:
    print("Error finding last_name")
except TypeError:
    print("Illegal add operation")

Catch all exceptions with except Exception:. This will handle any exception that comes its way. Generally you do not want to catch general exceptions, you want to catch specific exceptions as a matter of best practice.

You can access the error object this way:

try:
    last_name = student["last_name"]
except KeyError as error:
    print("Error finding last_name")
    print(error)

This does not give you access to the full stack trace, for that you would need to use the python traceback module. This will be covered in a later section.

You can also raise your own exception, create any exception you want, and use a finally handler after any exception that may occur.

Other Data Types

Overview of other data types in python:

complex
- Complex numbers
long
- Only in Python 2
- Replaced by integer in Python 3
byes
- Sequence of integers in the range of 0..255
- Sequence of strings or other objects
bytearray
- Similar to bytes
tuple
- Similar to lists
- Are immutable
set and frozenset
- Similar to lists, but only have unique objects
- Can be used to eliminate duplicate elements in a list: set([3, 2, 3, 1, 5]) == (1, 2, 3, 5)

Functions, Files, Yield, Lambda

def keyword is used to define a function:

students = []

def add_student(name):
    students.append(name)

return can be used to return values from a function:

def get_students_titlecase():
    students_titlecase = []
    for student in students:
        students_titlecase.append(student.title())
    return students_titlecase


student_list = get_students_titlecase()

If a return value is not specified, None is returned.

It is convention to follow function definitions with 2 newlines.

Function Arguments

Arguments are scoped to the function. Arguments are required by default and the Python parser will throw an exception if the arguments specified by the function definition are not provided in the function call.

In order to make an argument optional you must provide a default value:

students = []

def add_student(name, student_id=332):
    students.append({ "id": student_id, "name": name })


add_student("Mark") # Will not throw an exception

Named arguments in calling a function specifies the name of the arguments and may lead to legibility for developers:

add_student(name="Mark", student_id=15)

A function can be defined to accept a variable number of arguments using *:

def var_args(name, *args):
    print(name, args)


var_args("Chi-chi", "Is Learning Python", None, "Hello, World", True)
# Chi-chi ('Is Learning Python', None, 'Hello, World', True)

In this setup the arguments are stored in a list and must be iterated over to parse through the contents.

Keyword arguments can be used to define a variable number of named arguments using **:

def var_kwargs(name, **kwargs):
    print(name, kwargs)


var_kwargs("Mark", id=15, description="Python Student", feedback=None, pluralsight_subscriber=True)
"""
Mark {
    'id': 15,
    'description': 'Python Student',
    'feedback': None,
    'pluralsight_subscriber': True
}
"""

Keyword arguments store the named arguments as a dictionary and can be accessed as such.

Input Function

Python has a built-in input function to allow users to input values on the command line:

def print_args(**kwargs):
    print(kwargs)


name = input("Name: ")
id = input("ID: ")

print_args(name=name, id=id)
"""
Name: Mark
ID: 15
{'name': 'Mark', 'id': '15'}
"""

Nested Functions

You can nest functions inside of other functions to avoid polluting scope:

def get_students():
    students = ["mark", "james"]
    def get_students_titlecase():
        students_titlecase = []
        for student in students:
            students_titlecase.append(student.title())
        return students_titlecase
    students_titlecase_names = get_students_titlecase()
    print(students_titlecase_names)

Function closures exist in Python. Nested inner functions have access to variables defined in their outer functions.

Basic File Operations

Python's built in open function can be used to access files and perform operations:

def save_file(filename, text):
    try:
        f = open(f"{filename}.txt", "a") # "a" is an access mode argument
        f.write(text + "\n")
        f.close()
    except Exception:
        print("Could not save file")

Access modes in Python:

flag	mode
"w"	write: overwrites the entire file
"r"	read: read a text file
"a"	append: append text to a new or existing file
"rb"	read binary: read a binary file
"wb"	write binary: write to a binary file

Other IO operations in Python.

Reading a file in Python:

def read_file(filename):
    try:
        f = open(filename, "r")
        text = f.read()
        f.close()
        return text
    except Exception:
        print("Could not read file")

It is always a good idea to wrap any file operations in a try-except block.

Generator Functions: Yield

Resource: Generators - Python Wiki

Example of building an iterator (generator pattern):

class first_n(object):
    def __init__(self, n):
        self.n = n
        self.num, self.nums = 0, []

    def __iter__(self):
        return self

    # Python 3 compatibility
    def __next__(self):
        return self.next()

    def next(self):
        if self.num < self.n:
            cur, self.num = self.num, self.num + 1
            return cur
        else:
            raise StopIteration()

sum_of_first_n = sum(first_n(1000))

Negatives of the above pattern:

There is a lot of boilerplate
The logic has to be expressed in a convoluted way

Python provides generator functions as a shortcut to building iterators:

def first_n(n):
    num = 0
    while num < n:
        yield num
        num += 1

sum_of_first_n = sum(first_n(1000))

Lambda Functions

Lambda function notation is supported in Python:

# Standard python function notation:
def double(x):
    return x * 2

# Lambda function notation:
double = lambda x: x * 2

Lambda functions are simple 1-liners in Python.

Lambda functions are useful in higher-order functions: functions that take another function as an argument.

Object Oriented Programming

Python is an object-oriented language. While you are not required to use classes, Python provides them.

There is a debate about whether Python is truly an object-oriented language because it does not provide encapsulation (class methods are not private and hidden from the consumer).

There are no private methods. Most programmers prefix a method with _ or __ to indicate a method should not be accessed directly.

Classes

A logical group of functions and data.

Defining a class:

class Student:
    pass  # in Python pass is a no-op

Creating a new instance of a class Student:

student = Student()

Defining a class with instance and class attributes and methods:

class Student:

    school_name = "Springfield Elementary"

    def __init__(self, name, id):
        self.name = name
        self.id = id

    def __str__(self):
        retrun f"<{self.school_name}> #{self.id}: {self.name}"

mark = Student("Mark", 115)
print(mark) # <Springfield Elementary> #115: Mark

Notes:

__init__ is the constructor for classes
All class methods receive self as their first argument
- This is a reference to the class instance
__str__ is the built-in to-string method for a class
Assign values to self to create instance attributes
Class attributes are created by assigning variables inside the class body, but outside of methods
- Also known as static variables
- These are shared across all instances of the class
- Must be accessed via self inside of methods (ex: self.school_name)
- Can be directly accessed off of the class itself: Student.school_name

Inheritance and Polymorphism

Defining a child class:

class HighSchoolStudent(Student):

    school_name = "Springfield High"

    def __str__(self):
        return f"[self.school_name] #{self.id}: {self.name}"

    def primary_school(self):
        return super().school_name

james = HighSchoolStudent("James", 312)
print(james) # [Springfield High] #312: James
print(james.primary_school()) # Springfield Elementary

Notes:

Pass the parent class into the class definition to create a child class
Override parent methods by defining the same method on the child class
super() can be used inside a class method to access the parent class
- Methods and attributes on the parent class are accessed from the return value: super().school_name

Modules

To import from another file simply import the filename:

import hs_student

james = hs_student.HighSchoolStudent("James")

To import just one class from a module:

from hs_student import HighSchoolStudent

james = HighSchoolStudent("James")

To import all named entities from a module:

from hs_student import *

james = HighSchoolStudent("James")

You can import from your own modules as well as standard and third-party libraries.

Comment Block Convention

Annotating code:

students = []

def add_student(name, id):
    """
    Adds a student dictionary to the students list
    :param name: string - student name
    :param id: integer - student id number
    """

    students.append({ name: name, id: id, school: "Springfield Elementary" })

It is convention to annotate your functions and classes using multi-line string blocks. Provide a space between the comment and the function implementation.

Installng Python Packages

Install Python packages with pip. They are then available for any Python program run with the version and instance of Python that pip installed the dependency to.

Flask

Flask is a simple package for creating web servers. Flask can render html as well as html containing a templating language called Jinja2.

Tips And Tricks

Virtual Environments

Virtual environments allow you to set up independent Python environments when working with your Python application. This allows you to install different versions of the same depenencies in isolated environments.

By default pip will only install depdencies globally, which can make running different applications that use different versions of the same dependencies impossible without virutal environments.

From stackoverflow:

PyPI packages not in the standard library

virtualenv is a very popular tool that creates isolated Python environments for Python libraries. If you're not familiar with this tool, I highly recommend learning it, as it is a very useful tool, and I'll be making comparisons to it for the rest of this answer.

It works by installing a bunch of files in a directory (eg: env/), and then modifying the PATH environment variable to prefix it with a custom bin directory (eg: env/bin/). An exact copy of the python or python3 binary is placed in this directory, but Python is programmed to look for libraries relative to its path first, in the environment directory. It's not part of Python's standard library, but is officially blessed by the PyPA (Python Packaging Authority). Once activated, you can install packages in the virtual environment using pip.

pyenv is used to isolate Python versions. For example, you may want to test your code against Python 2.6, 2.7, 3.3, 3.4 and 3.5, so you'll need a way to switch between them. Once activated, it prefixes the PATH environment variable with ~/.pyenv/shims, where there are special files matching the Python commands (python, pip). These are not copies of the Python-shipped commands; they are special scripts that decide on the fly which version of Python to run based on the PYENV_VERSION environment variable, or the .python-version file, or the ~/.pyenv/version file. pyenv also makes the process of downloading and installing multiple Python versions easier, using the command pyenv install.
pyenv-virtualenv is a plugin for pyenv by the same author as pyenv, to allow you to use pyenv and virtualenv at the same time conveniently. However, if you're using Python 3.3 or later, pyenv-virtualenv will try to run python -m venv if it is available, instead of virtualenv. You can use virtualenv and pyenv together without pyenv-virtualenv, if you don't want the convenience features.
virtualenvwrapper is a set of extensions to virtualenv (see docs). It gives you commands like mkvirtualenv, lssitepackages, and especially workon for switching between different virtualenv directories. This tool is especially useful if you want multiple virtualenv directories.
pyenv-virtualenvwrapper is a plugin for pyenv by the same author as pyenv, to conveniently integrate virtualenvwrapper into pyenv.
pipenv by Kenneth Reitz (the author of requests), is the newest project in this list. It aims to combine Pipfile, pip and virtualenv into one command on the command-line. The virtualenv directory typically gets placed in ~/.local/share/virtualenvs/XXX, with XXX being a hash of the path of the project directory. This is different from virtualenv, where the directory is typically in the current working directory.

The Python Packaging Guide recommends pipenv when developing Python applications (as opposed to libraries). There does not seem to be any plans to support venv instead of virtualenv (#15). Confusingly, its command-line option --venv refers to the virtualenv directory, not venv, and similarly, the environment variable PIPENV_VENV_IN_PROJECT affects the location of the virtualenv directory, not venv directory (#1919).

Standard library

pyvenv is a script shipped with Python 3 but deprecated in Python 3.6 as it had problems (not to mention the confusing name). In Python 3.6+, the exact equivalent is python3 -m venv.
venv is a package shipped with Python 3, which you can run using python3 -m venv (although for some reason some distros separate it out into a separate distro package, such as python3-venv on Ubuntu/Debian). It serves a similar purpose to virtualenv, and works in a very similar way, but it doesn't need to copy Python binaries around (except on Windows). Use this if you don't need to support Python 2. At the time of writing, the Python community seems to be happy with virtualenv and I haven't heard much talk of venv.

Most of these tools complement each other. For instance, pipenv integrates pip, virtualenv and even pyenv if desired. The only tools that are true alternatives to each other here are venv and virtualenv.

Recommendation for beginners

This is my personal recommendation for beginners: start by learning virtualenv and pip, tools which work with both Python 2 and 3 and in a variety of situations, and pick up the other tools once you start needing them.

Debugging Python Code

Python 3.7 introduces a new breakpoint() function through the standard library pdb. For more details check the quick tour post on Hackernoon.

Creating Executable Files

Executables of your Python applications can be created using pyinstaller.

To create an exe file, simply enter into the command line while in your project directory:

$ pyinstaller main.py

Replace main.py with your own entrypoint file.

Pyinstaller will create a /build and /dist directory, as well as a main.spec file, in your project directory. Inside of the /dist/main directory you will find a main.exe file.

Building your project this way generates a number of .pyd and .dll files in your /dist directory that are necessary to run your new executable. In order to bundle everything into a single .exe file run the following in your command line:

$ pyinstaller --onefile main.py

The --onefile flag tells pyinstaller to bundle everything into a single executable. When you run pyinstaller with this flag, the /dist directory will only contain a single file: main.exe.

This executable file will be very large. This is because the Python interpreter along with required Python packages have to be bundled in with this executable.

Creating A Setup Wizard

Distributing just an .exe file can be a bit strange. Often you will want to provide an installer which will install the program into the Program Files directory and add it to the start up menu as well. You may also want to include additional files and assets along with your program (help file, README, images, etc).

Inno Setup is a Windows program that will help you create a setup wizard for any program.

Files

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