Software engineering principles, from Robert C. Martin's book Clean Code, adapted for Python. This is not a style guide. It's a guide to producing readable, reusable, and refactorable software in Python.
Not every principle herein has to be strictly followed, and even fewer will be universally agreed upon. These are guidelines and nothing more, but they are ones codified over many years of collective experience by the authors of Clean Code.
Inspired from clean-code-javascript
Targets Python3.7+
Bad:
ymdstr = datetime.date.today().strftime("%y-%m-%d")Good:
current_date: str = datetime.date.today().strftime("%y-%m-%d")Bad: Here we use three different names for the same underlying entity:
get_user_info()
get_client_data()
get_customer_record()Good: If the entity is the same, you should be consistent in referring to it in your functions:
get_user_info()
get_user_data()
get_user_record()Even better Python is (also) an object oriented programming language. If it makes sense, package the functions together with the concrete implementation of the entity in your code, as instance attributes, property methods, or methods:
class User:
info : str
@property
def data(self) -> dict:
# ...
def get_record(self) -> Union[Record, None]:
# ...We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.
Bad:
# What the heck is 86400 for?
time.sleep(86400);Good:
# Declare them in the global namespace for the module.
SECONDS_IN_A_DAY = 60 * 60 * 24
time.sleep(SECONDS_IN_A_DAY)Bad:
address = 'One Infinite Loop, Cupertino 95014'
city_zip_code_regex = r'^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$'
matches = re.match(city_zip_code_regex, address)
save_city_zip_code(matches[1], matches[2])Not bad:
It's better, but we are still heavily dependent on regex.
address = 'One Infinite Loop, Cupertino 95014'
city_zip_code_regex = r'^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$'
matches = re.match(city_zip_code_regex, address)
city, zip_code = matches.groups()
save_city_zip_code(city, zip_code)Good:
Decrease dependence on regex by naming subpatterns.
address = 'One Infinite Loop, Cupertino 95014'
city_zip_code_regex = r'^[^,\\]+[,\\\s]+(?P<city>.+?)\s*(?P<zip_code>\d{5})?$'
matches = re.match(city_zip_code_regex, address)
save_city_zip_code(matches['city'], matches['zip_code'])Don’t force the reader of your code to translate what the variable means. Explicit is better than implicit.
Bad:
seq = ('Austin', 'New York', 'San Francisco')
for item in seq:
do_stuff()
do_some_other_stuff()
# ...
# Wait, what's `item` for again?
dispatch(item)Good:
locations = ('Austin', 'New York', 'San Francisco')
for location in locations:
do_stuff()
do_some_other_stuff()
# ...
dispatch(location)If your class/object name tells you something, don't repeat that in your variable name.
Bad:
class Car:
car_make: str
car_model: str
car_color: strGood:
class Car:
make: str
model: str
color: strTricky
Why write:
def create_micro_brewery(name):
name = "Hipster Brew Co." if name is None else name
slug = hashlib.sha1(name.encode()).hexdigest()
# etc.... when you can specify a default argument instead? This also makes ist clear that you are expecting a string as the argument.
Good:
def create_micro_brewery(name: str = "Hipster Brew Co."):
slug = hashlib.sha1(name.encode()).hexdigest()
# etc.Limiting the amount of function parameters is incredibly important because it makes testing your function easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.
Zero arguments is the ideal case. One or two arguments is ok, and three should be avoided. Anything more than that should be consolidated. Usually, if you have more than two arguments then your function is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument.
Bad:
def create_menu(title, body, button_text, cancellable):
# ...Good:
class Menu:
def __init__(self, config: dict):
title = config["title"]
body = config["body"]
# ...
menu = Menu(
{
"title": "My Menu",
"body": "Something about my menu",
"button_text": "OK",
"cancellable": False
}
)Also good
class MenuConfig:
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig):
title = config.title
body = config.body
# ...
config = MenuConfig
config.title = "My delicious menu"
config.body = "A description of the various items on the menu"
config.button_text = "Order now!"
# The instance attribute overrides the default class attribute.
config.cancellable = True
create_menu(config)Fancy
from typing import NamedTuple
class MenuConfig(NamedTuple):
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig):
title, body, button_text, cancellable = config
# ...
create_menu(
MenuConfig(
title="My delicious menu",
body="A description of the various items on the menu",
button_text="Order now!"
)
)Even fancier
from dataclasses import astuple, dataclass
@dataclass
class MenuConfig:
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig):
title, body, button_text, cancellable = astuple(config)
# ...
create_menu(
MenuConfig(
title="My delicious menu",
body="A description of the various items on the menu",
button_text="Order now!"
)
)This is by far the most important rule in software engineering. When functions do more than one thing, they are harder to compose, test, and reason about. When you can isolate a function to just one action, they can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.
Bad:
def email_clients(clients: List[Client]):
"""Filter active clients and send them an email.
"""
for client in clients:
if client.active:
email(client)Good:
def get_active_clients(clients: List[Client]) -> List[Client]:
"""Filter active clients.
"""
return [client for client in clients if client.active]
def email_clients(clients: List[Client, ...]) -> None:
"""Send an email to a given list of clients.
"""
for client in clients:
email(client)Do you see an opportunity for using generators now?
Even better
def active_clients(clients: List[Client]) -> Generator[Client]:
"""Only active clients.
"""
return (client for client in clients if client.active)
def email_client(clients: Iterator[Client]) -> None:
"""Send an email to a given list of clients.
"""
for client in clients:
email(client)Bad:
class Email:
def handle(self) -> None:
# Do something...
message = Email()
# What is this supposed to do again?
message.handle()Good:
class Email:
def send(self) -> None:
"""Send this message.
"""
message = Email()
message.send()When you have more than one level of abstraction, your function is usually doing too much. Splitting up functions leads to reusability and easier testing.
Bad:
def parse_better_js_alternative(code: str) -> None:
regexes = [
# ...
]
statements = regexes.split()
tokens = []
for regex in regexes:
for statement in statements:
# ...
ast = []
for token in tokens:
# Lex.
for node in ast:
# Parse.Good:
def parse_better_js_alternative(code: str) -> None:
tokens = tokenize(code)
syntax_tree = parse(tokens)
for node in syntax_tree:
# Parse.
def tokenize(code: str) -> list:
REGEXES = [
# ...
]
statements = code.split()
tokens = []
for regex in REGEXES:
for statement in statements:
# Append the statement to tokens.
return tokens
def parse(tokens: list) -> list:
syntax_tree = []
for token in tokens:
# Append the parsed token to the syntax tree.
return syntax_treeFlags tell your user that this function does more than one thing. Functions should do one thing. Split your functions if they are following different code paths based on a boolean.
Bad:
from pathlib import Path
def create_file(name: str, temp: bool) -> None:
if temp:
Path('./temp/' + name).touch()
else:
Path(name).touch()Good:
from pathlib import Path
def create_file(name: str) -> None:
Path(name).touch()
def create_temp_file(name: str) -> None:
Path('./temp/' + name).touch()A function produces a side effect if it does anything other than take a value in and return another value or values. For example, a side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.
Now, you do need to have side effects in a program on occasion - for example, like in the previous example, you might need to write to a file. In these cases, you should centralize and indicate where you are incorporating side effects. Don't have several functions and classes that write to a particular file - rather, have one (and only one) service that does it.
The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, or using an instance of a class, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.
Bad:
# Global variable referenced by following function.
# If another function used this name, now it'd be an array and could break.
name = 'Ryan McDermott'
def split_into_first_and_last_name() -> None:
global name
name = name.split()
split_into_first_and_last_name()
print(name) # ['Ryan', 'McDermott']Good:
def split_into_first_and_last_name(name: str) -> None:
return name.split()
name = 'Ryan McDermott'
new_name = split_into_first_and_last_name(name)
print(name) # 'Ryan McDermott'
print(new_name) # ['Ryan', 'McDermott']Also good
class Person:
name: str
def __init__(self, name: str):
self.name = name
@property
def name_as_first_and_last(self) -> list:
return self.name.split()
person = Person('Ryan McDermott')
print(person.name) # 'Ryan McDermott'
print(person.name_as_first_and_last) # ['Ryan', 'McDermott']