Software development is complicated and often requires significant flexibility in terms of the patterns used. But... Using that flexibility in all cases results in difficult to read and buggy code. The topics covered here are intended to provide good default patterns to use in most cases. In exceptional cases, other patterns can be discussed and considered.
The goal of defining and applying these practices is to increase code quality when the code is first written. This is expected to improve product quality while also decreasing development time.
When you want to make sure you do something later, context managers are the answer.
The with statement is how you use a context manager.
This is useful for many forms of resource management and cleanup.
- Closing files that are opened
- Releasing locks that are acquired
- Committing or rolling back database transactions
- Shutting down services that are started
- Deleting temporary files that are created
- etc
One common use of a context manager is to encapsulate a try: block in a reusable form.
The underlying dunder (double underscore) methods for a context manager are .__enter__() and .__exit__().
Usually it is more natural to use @contextlib.contextmanager to create a context manager from a generator function.
Async context managers can be similarly created for use with the async with statement.
Context managers can often also replace functions that accept callback functions which are called immediately while providing some setup or teardown. The context manager has the upside that the 'callback function' can be just bare code and also that the manager doesn't need to have any awareness of the code it is providing setup and teardown for.
Note that you can already use a file object as a context manager so these definitions are already available directly on the file, but it makes a simple example.
from contextlib import contextmanager
from pathlib import Path
from typing import Callable, TextIO, TypeVar
T = TypeVar("T")
# just manually do it without any re-usability
def manual(path: Path):
file = open(path)
try:
return file.read()
finally:
file.close()
# use a function that takes a callback
def callbacker(path: Path, callback: Callable[[TextIO], T]) -> T:
file = open(path)
try:
return callback(file)
finally:
file.close()
def the_callback(file: TextIO) -> str:
return file.read()
def use_callbacker(path: Path) -> str:
return callbacker(path=path, callback=the_callback)
# the preferred form for this example with a context manager
@contextmanager
def manager(path: Path):
file = open(path)
try:
yield file
finally:
file.close()
def use_managed(path: Path):
with manager(path=path) as file:
return file.read()
# the real form for this since files are already context managers
def use_open_directly(path: Path):
with open(path) as file:
return file.read()Even when no reuse is necessary there are still reasons to use context managers. They allow encapsulation of the setup and teardown separately from the code using it. In doing so they help avoid mixing with other setup and teardown resulting in unintended consequences.
def f(x, y, z):
x.setup()
y.setup(x)
try:
z.process(x, y)
finally:
x.teardown()
y.teardown()This example has some likely errors and risks.
Normally you teardown in the opposite order that you setup while this code does not.
If y.setup() fails then x.teardown() will not get called.
If x.teardown() fails then y.teardown() will not get called.
These errors would not be possible as written below.
def f(x, y, z):
with x.setup():
with y.setup(x):
z.process(x, y)- https://github.com/Cactus-Network/cactus-blockchain/pull/11467
- https://github.com/Cactus-Network/cactus-blockchain/pull/10166
There are a few basic goals for classes that are targeted by the guidance provided below.
- Creating an instance of a class should be a trivial activity so that when needed it can be done without any expensive computation, waiting on other resources, reading from disk, or having to bypass any restrictive checks.
- The attributes of a class should always be present from the moment the instance exists until the last reference is dropped.
- The list of attributes should be clearly apparent at a glance including detailed type hints.
One useful tool in encouraging and helping achieve the above goals is the dataclasses module.
It is useful for the vast majority of classes, not just trivial classes with a few attributes and no methods.
One of the benefits of using a more structured form of class definition is to take away flexibility in favor of quickly understandable consistency.
Here is the basic form of a class written using dataclasses.
from dataclasses import dataclass
@dataclass(frozen=True)
class Developer:
name: str
words_per_minute: float
primary_language: strWhen we avoid using @dataclass it is easy to make various errors.
There are two places where you might add hints: on the class itself and in .__init__().
It is easy to miss hints altogether.
Some attributes may only be assigned in some cases.
All of these make it more complicated to write a quality class and harder to read the result even when it is correct.
While it is important to be able to construct an instance of a class trivially, there are often expensive operations that coincide with instantiation in many cases.
Since there is only one .__init__(), we must use regular functions and @classmethods to move forward with supporting the non-trivial cases of construction.
This is often a foreign and troublesome concept to accept.
Let's consider a case with a builtin type where not doing this would be clearly problematic.
In a case where you want a random integer, would you rather write random.randrange(10) or int(random=True, maximum=10)?
For another way we like to create integers, do we want int.from_bytes(blob, byteorder="big") or int(from_bytes=True, byteorder="big").
If you consider the number of parameters that would have to be mixed together in int.__init__() to handle all the possible cases it quickly becomes apparent that using int() for all means of creating an integer is not practical.
Luckily, most classes don't do that and ours don't have to either.
How about a couple examples.
import json
from dataclasses import dataclass
from typing import Type, TypeVar
_T_Coin = TypeVar("_T_Coin", bound="Coin")
@dataclass(frozen=True)
class Coin:
hash: bytes
value: int
@classmethod
def from_json(cls: Type[_T_Coin], text: str) -> _T_Coin:
decoded = json.loads(text)
return cls(
hash=bytes.fromhex(decoded["hash"]),
value=decoded["value"],
)
def create_coin_from_bytes(blob: bytes) -> Coin:
return Coin(
hash=blob[:32],
value=int.from_bytes(blob[32:], byteorder="big"),
)In many cases there isn't much need to modify a class.
This comes with benefits like knowing that an object won't be modified when you are using it.
Inline with the above recommendations there is @dataclass(frozen=True) for this.
While Python offers no true guarantees of immutability, using frozen=True will result in an exception if you try to assign a new value to an attribute using any normal means.
This is a good default to start with until you find the need to mutate the instances.
Note that this does not stop you from mutating an attribute object itself.
my_frozen_instance.a_list_attribute.append(23) is still possible.
Keep this in mind when considering what types to use as attributes on frozen classes.
In some languages inheritance is the only way to mix multiple types of objects into a single container. That is not the case with Python. We should strive to learn about other options and figure out how to make them work for us.
Inheritance is somewhere between hard and impossible to do right.
At nearly all cost, do not hint Any...
Yes, before you know anything about type hinting, know that.
Any indicates that you can do anything you want with the hinted object which basically defeats the use of type checking.
If you don't care what type the object is, use object.
Now that you know how to avoid defeating type hint checks...
Python is strongly and dynamically typed. Being strongly typed means that an object isn't just an object with attributes, it is an instance of a specific type. Being dynamically typed means that a variable, an attribute, a list element, or any other reference can refer to an object that is an instance of any type and that can change at any assignment. While not being constrained to strictly only referencing particular types can be useful, it can also encourage impossible to follow code. It is often helpful to default to not leveraging this flexibility. Type hints are used to indicate the intended level of flexibility at each point in the code. Objects are not checked against the type hints at runtime. mypy is used to statically analyze the hints.
Clearly defining what types can be properly handled in each function parameter, variable, and attribute allows for static analysis to check for errors basic errors. Passing a list where a set is expected, returning a float where an int is expected, indexing a list with a string because you thought it was a dict, and so on. While tests are still critical to confirm that the proper values are created, hinting makes it easy to get extensive coverage that proper types are being processed. Our goal is to reach complete hinting coverage on all code with relatively strict mypy configuration. This provides a rigorous level of checking for basic errors such as accessible unavailable attributes, mixing different types of elements in lists, treating a list as if it is a dict, and so on.
Note that as of Python 3.9 there were some broad changes implemented that allowed for hinting with the builtin list[int] as opposed to typing.List[int].
Also, in new Python versions Union[str, int] can be written as str | int.
While it is possible to some degree for us to use those forms despite running in older Python versions, we will not presently be using the newer forms.
Yes. If it is hard to hint, it is often hard to reason about and this should be taken as encouragement to explore simpler alternative forms for the code. If it is hard to deal with the knock on complaints from mypy triggered by the hints, then again, maybe there's another better form for the code.
from typing import List
def sum_bigger_values(values: List[int], minimum: int) -> int:
return sum(value for value in values if value > minimum)This says that our function accepts a parameter values that is hinted as List[int] meaning it will be a list where each element is an integer.
The minimum parameter should be an integer.
-> int: indicates that the returned value will be an integer.
Note that Python functions always return a single object.
If there is either no return statement or just a bare return statement, the None object is returned.
Regardless, this should be hinted explicitly with -> None:.
While a bit verbose, this is simpler than making exceptions for the right cases while not letting the wrong cases slip through.
All Python references always refer to an object.
Sometimes we want to indicate that there presently is no object for them to refer to.
Often we use the None object for these cases.
The term 'optional' is used to express this.
from typing import Optional
def print_name(first: str, last: Optional[str] = None) -> None:
if last is None:
return first
return f"{first} {last}"The last parameter can be passed either a string or None.
Note that an optional parameter is distinct from an optional hint, though they are often used together.
An optional parameter has a default and is not required to be passed when calling the function.
For example, def f(x: int = 0) -> None: can be called as f() without passing the optional x parameter.
Since it is not hinted as optional, f(x=None) is not valid from a type hinting perspective.
In the other direction, def g(x: Optional[int]) -> None may be called as g(x=None) since x is hinted as Optional, but g() is not valid since the parameter x has no default value.
Note that Optional[int] is equivalent to Union[None, int].
Sometimes in error messages from mypy you will see the Union form even when you typed the Optional form.
While optional hinted parameters aren't too much trouble, optional returns and attributes have significant implications.
Every location that calls a function with an optional return or accesses an optionally hinted attribute is likely to have to if value is None: or similar before acting on the value.
If we consider a hint such as Optional[List[int]], in many cases it will be simpler to just hint List[int] and have an empty list instead of None.
Another option is to raise an exception instead of returning None.
The best choice is highly dependent on the context in which the function is called and how the result is handled.
Please consider all options against both the concept of the function and the pragmatic usage of the function.
In some cases it will take a non-trivial code reorganization to avoid the optionality.
With new code, take that seriously.
With existing code, at least try to see what the alternative form would be as a chance to practice thinking it through.
Sometimes code can handle multiple different types despite them not being in a common inheritance hierarchy. Unions describe these cases when the types are just different.
from os import PathLike
from pathlib import Path
from typing import Union
def read_file(path: Union[str, PathLike]) -> str:
return Path(path).read_text(encoding="utf-8")In other cases of allowing multiple types that are also not related by inheritance but which do provide similar attributes and methods, interfaces can be used.
The preferred mechanism for defining object interfaces is Protocol.
This enables structural typing where the type hints can require certain attributes of the object instead of operating purely based on an inheritance hierarchy to determine compatibility.
Since inheritance isn't required, this can be retrofitted to existing purely duck-typed code without having to actually modify every relevant class in every repo.
Until we drop Python 3.7 support we will need to get the Protocol class from typing_extensions instead of from typing.
from dataclasses import dataclass
from typing_extensions import Protocol
class WaterfowlProtocol(Protocol):
species: str
def vocalize(self, count: int) -> str:
...
def bother_waterfowl(fowl: WaterfowlProtocol, aggressiveness: int):
print(f"the {fowl.species} went {fowl.vocalize(count=aggressiveness)}")
@dataclass
class Duck:
species: str = "duck"
def vocalize(self, count: int) -> str:
return " ".join(["quack"] * count)
@dataclass
class Goose:
species: str = "goose"
def vocalize(self, count: int) -> str:
return " ".join(["honk"] * count)
bother_waterfowl(fowl=Duck(), aggressiveness=1)
bother_waterfowl(fowl=Goose(), aggressiveness=3)When hinting based on a Protocol there can often be unexpected complexities.
Consider passing in an instance of a class with an attribute hinted str while the protocol hints it as Union[str, int].
It is common to expect that str satisfies Union[str, int] and to be throughly confused by mypy's complaint that it does not.
The hazard here is that since the function receiving the object thinks the attribute can be either a str or an int it may decide to assign an int to it.
The protocol says this is ok.
A good solution for many cases is to hint that attribute as being read only on the protocol.
This is done via a read only property.
Note that attributes of protocols that are themselves protocols should be read-only (python/mypy#12990).
from typing import Union
from typing_extensions import Protocol
class AProtocol(Protocol):
a_writable_attribute: bool
@property
def a_read_only_attribute(self) -> Union[str, int]:
...Aside from identifying attributes on a class, Protocol can be used for more expressive hinting of callables than you can do with Callable.
One use is to be able to indicate parameter names.
Remember that functions and methods are just themselves regular Python objects.
When you call an object such as calling f by writing f(), the .__call__() method is what gets executed.
A protocol for a callable often has just a .__call__() method.
from typing_extensions import Protocol
class ThreeIntAdder(Protocol):
def __call__(self, first: int, second: int, third: int) -> int:
...Another option for additional expressivity around hinting a callable with a protocol is to use overloads to narrow the possible combinations of calls. It is often better to just avoid overload situations, but as we retrofit hints to existing code we may prefer this option sometimes.
TypeVar allows you to create 'variables' for type hints.
While a regular hint indicates something about a single element that you are hinting, a TypeVar is used to indicate a relationship between multiple elements being hinted.
They are not meant for basic individual element hinting, at least not generally.
from typing import TypeVar
T = TypeVar("T")
def double(original: T) -> T:
return 2 * original
an_int = double(original=2)
a_list = double(original=["a", "b"])In this case we have related the parameter original indicating that it will be the same type as the return value will be.
If you pass in an int you will get an int back.
If you pass in a list you will get a list back.
TypeVar is not a good tool for relating multiple parameter types to each other.
from typing import TypeVar
T = TypeVar("T")
def add(this: T, that: T) -> T:
return this + that
an_int = add(this=3, that=9)
an_object = add(this="a", that=2)What is happening here is that mypy will see that there are two parameters using the TypeVar and it will find the common ancestor of their types.
When passing an int and an int the common ancestor is int so you get that back as you might like to.
When passing a str and an int, they both directly inherit from object so that is the common ancestor and so that is the return type.
You probably intended to get an error in this case.
Probably don't do this.
While not the most basic sort of hint, it is pretty common to see something like List[str].
This just says that there will be a list, and it will contain strings.
List here is a generic.
While useful in other contexts, the use here of having a generic container where you can describe what it holds is an easy way to get started with generics.
Let's step it up a notch and consider a dictionary with string keys mapping to integer values, Dict[str, int].
With this information mypy is able to know what sorts of objects will be in the result of .keys() or .values().
Or that in for key, value in the_dict.items():, key will be a str and value will be an int.
When you look at defining a generic it becomes mostly about relating type hints in different places to each other. Without writing an implementation, let's see what part of a cache leveraging generics might look like.
from dataclasses import dataclass, field
from typing import Dict, Generic, Optional, TypeVar
KT = TypeVar("KT")
VT = TypeVar("VT")
@dataclass
class Cache(Generic[KT, VT]):
_mapping: Dict[KT, VT] = field(default_factory=dict)
def get(self, key: KT, default: Optional[VT] = None) -> Optional[VT]:
...
def set(self, key: KT, value: VT) -> None:
...
c = Cache[int, str]()
# error: Argument 1 to "get" of "Cache" has incompatible type "str"; expected "int"
c.get("abc")
# error: Incompatible types in assignment (expression has type "Optional[str]", variable has type "bytes")
x: bytes = c.get(3)Occasionally you need to hint a thing that does not exist yet. This may occur when defining a class where a method needs to hint the class itself. Here is a failing example.
from typing_extensions import final
@final
class C:
@classmethod
def create(cls) -> C:
return cls()This results in a NameError.
Traceback (most recent call last):
File "/home/altendky/tmp/x.py", line 4, in <module>
class C:
File "/home/altendky/tmp/x.py", line 6, in C
def create(cls) -> C:
NameError: name 'C' is not definedThe definition of class C has not been completed yet so the resulting object has not been assigned to the name C yet, hence the NameError.
This can be avoided for the Python runtime case by quoting as -> "C":.
from typing_extensions import final
@final
class C:
@classmethod
def create(cls) -> "C":
return cls()When you either run mypy or use typing.get_type_hints(), this string "C" will get resolved to the class itself.
An alternative that does not require the quotes is to use the special import from __future__ import annotations.
This is the recommended form.
from __future__ import annotations
from typing_extensions import final
@final
class C:
@classmethod
def create(cls) -> C:
return cls()This behavior was going to become default in Python 3.10 but was removed and did not make it into Python 3.11 either.
See the footnote on the __future__ doc page.
If you want to read more about this see PEP 563 and PEP 649.
A more complicated case can come about from circular imports that are only relevant to hinting, not to runtime code.
Avoiding the circular import at runtime is done by 'hiding' the problematic import in an if TYPE_CHECKING: block.
At runtime, Python will consider that false and ignore it.
When running mypy analysis it will consider it true and process the imports.
mypy doesn't suffer from the circular import issues.
This does trigger the situation similar to above though where you try to reference a class which is not defined.
Both practices can be used together as in the example below to get to a complete solution.
from __future__ import annotations
from dataclasses import dataclass
from typing import TYPE_CHECKING
if TYPE_CHECKING:
# Note that the wallet state manager module imports the wallets.
# This would create a problematic circular import condition at
# runtime that `if TYPE_CHECKING:` avoids.
from cactus.wallet.wallet_state_manager import WalletStateManager
@dataclass
class SomeWallet:
wallet_state_manager: WalletStateManager- Do not import
test_*modules. Instead locate shared tooling in non-test files within thetests/directory or subdirectories. - Do not import fixtures. Fixtures are shared by locating them in
conftest.pyfiles at whatever directory layer you want them to be recursively available from. - Do not use test classes.
unittestrequires that tests be held in a class. pytest does not. Our tests are fully dependent on pytest so there's no use in retainingunittestcompatibility for this single point. Making a few testsunittestcompatible is not useful compared with the cost of inconsistency.
- Use Click.
- Use subcommands for separate activities.
- Don't make users write JSON generally.
- Short options should be a single character.
- Long options should delimit words by dashes, not underscores.
- But backwards compatibility and consistency with the existing not-this-way stuff, how do we handle that?
- Don't catch
CancelledError - Consider shielding cancellation in shutdown cleanup code, maybe
- Store references to all tasks you spawn and be sure to clean them up
- For delta timing within a process do not use
time.time()as it can have 'fake' deltas due to system clock changes.time.monotonic()is the direct alternative, though for specific cases other clocks tied to CPU performance, process time, or thread time may be of interest.
- Avoid use of non-booleans as booleans such as
if the_list:. If you meanif len(the_list) > 0:write that, if you meanif an_optional_thing is not None:write that.
Exceptions provide a somewhat secondary path through the code compared to the normal return path from functions.
This adds complexity when considering the program flow.
It also avoids error checking and propagation boilerplate in code that won't be handling the exceptions.
This in turn avoids accidentally forgetting to check for or propagate an error.
Sadly, there are no 'type hints for exceptions' in Python at this time.
Exception handling should be focused.
Catch only the specific exceptions you know how to handle properly at that specific location.
When catching exceptions, remember that only rarely have you thought through all the possible exceptions that could occur.
Consider that if you have made a typo such as pint("something") (note the r missing from print) you probably want to know about this immediately.
You want the code to fail quickly and clearly with a NameError, not silently continue on to doing other things as if some miscellaneous network connection error occurred, for example.
This is why linters discourage bare except: and overly broad except Exception: clauses.
Especially don't except BaseException: as that can consume even shutdown requests.
from datetime import datetime
class TooBigError(Exception):
pass
def maybe_add_two(x: int) -> int:
y = int(input("enter a number:"))
if y > 3:
raise TooBigError(f"{y} is too big!")
return x + y
value = 0
for value in range(5):
try:
value += maybe_add_two(x=value)
except TooBigError:
print("oh well, too big, whatever")
continue
date_string = datetime.now().isoformat(timespec="microseconds")
print(f"{date_string} value: {value}")This example shows a few aspects of focused exception handling. Let's compare it with the broad example below.
from datetime import datetime
def maybe_add_two(x: int) -> int:
y = int(input("enter a number:"))
if y > 3:
raise ValueError(f"{y} is too big!")
return x + y
value = 0
for value in range(5):
try:
value += maybe_add_two(x=value)
date_string = datetime.now().isoformat(timespec="microseconds")
print(f"{date_string} value: {value}")
except:
print("oh well, too big, whatever")In normal cases, these examples do the same thing.
In some exceptional cases, they do not.
One case where they differ is that the broad example has several lines in the try: block.
An exception from any of these lines, or functions they call, will trigger the except: block.
In the focused example, only the one line that we have considered handling the exception from can trigger the except TooBigError: block.
Additionally, in the focused example we catch only a single exception, TooBigError.
The broad example would also catch any NameErrors coming from typos, etc., which would make debugging those much more complicated.
Not only does the focused example only catch a single exception type, it catches a single exception type that we defined.
This puts us in control of what might raise it.
In the broad example, there are two separate points that could readily exercise this hole.
First, the int() of the input could raise a ValueError such as for an input of "m" which can't be parsed to an integer.
Second, the .isoformat() call can raise a ValueError for an invalid timespec= argument.
Neither of these are the "is too big!" exception we were intending to catch.
- Raise your own errors to avoid grouping with other exceptions
- Include as few lines as possible in the
try:block to avoid handling exceptions from other lines - Catch specific exceptions you have thought about how to handle
Note that deeply buried I/O is rich with both opportunities for exceptions to be raised and the types of exceptions.
Just imagine all the numerous ways that reading and writing from disk can fail.
Picking the proper exception can be difficult.
Sometimes OSError is a useful intermediately scoped exception to handle file not found, permissions errors, etc.
In some cases you want to respond to a specific exception but still have an exception propagate.
You may want the original exception to continue after your action, or to raise a new exception that is more descriptive.
When reraising the original exception use just raise instead of raise e.
This avoids the exception traceback looking like it came from the line where it was reraised.
When raising a new exception that is meant to replace the original, use raise TheException() from e.
This documents that the new exception didn't just happen to occur while handling the original, rather it is a more descriptive replacement for the original.
In either case, both tracebacks will be included, but they will describe themselves differently.
Either During handling of the above exception, another exception occurred: for a new raise TheException() or The above exception was the direct cause of the following exception: for raise TheException() from e.
class TooBigError(Exception):
pass
class InvalidInputError(Exception):
pass
def maybe_add_two(x: int) -> int:
y_raw = input("enter a number:")
try:
y = int(y_raw)
except ValueError as e:
print(f"unable to parse as an integer: {y_raw!r}")
raise InvalidInputError() from e
if y > 3:
raise TooBigError(f"{y} is too big!")
return x + yOn occasions where it is desired to handle a broad swath of exceptions, there are some specific considerations.
This should happen primarily at a high level such as in an RPC framework.
For example, the RPC framework may want to take any exception raised by the route handler code and turn it into a well-formed failure response with "success": False, the error, and the traceback.
A corner case around such places where you might except Exception as e: is that in Python 3.7 and below asyncio.CancelledError inherits from Exception.
In Python 3.8 and above asyncio.CancelledError inherits from BaseException.
Cancellation should not be consumed except maybe at the highest levels.
To provide broad exception handling while not accidentally catching cancellation, the cancellation exception can be caught and raised first.
import asyncio
async def main():
try:
await asyncio.sleep(5)
except asyncio.CancelledError:
raise
except Exception as e:
print(e)- Categorize information into groups
- mnemonics
- local usernames
- coin IDs
- etc
- Which groups can go in logs?
- Which groups can go in RPC responses?
- Which groups can go in diagnostic reports? (beta program, etc)
- use
-xwith cherry pick