Cheatsheets Python

35 entries

Python

Python reference covering built-ins, data structures, file I/O, comprehensions, and classes

35 commands

CommandDescriptionExample

len(obj)

Return the number of items in a sequence or collection

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

range(start, stop, step)

Generate a sequence of integers from start up to (not including) stop

list(range(0, 10, 2)) # [0,2,4,6,8]

enumerate(iterable)

Return (index, value) pairs when iterating over a sequence

for i, v in enumerate(['a','b']): ...

zip(a, b)

Combine two iterables into an iterator of tuples

list(zip([1,2], ['a','b'])) # [(1,'a'),(2,'b')]

map(fn, iterable)

Apply a function to every item in an iterable

list(map(str.upper, ['hi'])) # ['HI']

filter(fn, iterable)

Return items from an iterable for which the function returns True

list(filter(None, [0,1,'',2])) # [1,2]

sorted(iterable, key=fn, reverse=True)

Return a new sorted list with optional key function and order

sorted(names, key=len) # by length

isinstance(obj, type)

Check if an object is an instance of a class or type

isinstance(42, int) # True

lst.append(x)

Append an item to the end of a list

colors.append('blue')

lst.extend([x, y])

Extend a list by appending all items from an iterable

nums.extend([4, 5, 6])

lst.pop(index)

Remove and return the item at a given index (default: last)

last = stack.pop()

lst.sort(key=fn)

Sort a list in-place with an optional key function

words.sort(key=str.lower)

lst[1:4]

Slice a list from index 1 up to (not including) index 4

[10,20,30,40,50][1:4] # [20,30,40]

lst[::-1]

Reverse a list using slice notation

[1,2,3][::-1] # [3,2,1]

d.get('key', default)

Get a value by key, returning a default if the key doesn't exist

cfg.get('port', 8080)

d.items()

Return a view of (key, value) pairs in a dictionary

for k, v in user.items(): ...

d.update({'key': 'val'})

Update a dictionary with key-value pairs from another dict

config.update({'debug': True})

{**d1, **d2}

Merge two dictionaries into a new one (Python 3.5+)

{**defaults, **overrides}

d.setdefault('key', [])

Return the value for a key, setting it to a default if absent

groups.setdefault('admin', [])

str.split(' ')

Split a string into a list on a delimiter

'hello world'.split() # ['hello','world']

', '.join(lst)

Join a list of strings into a single string with a separator

', '.join(['a','b','c']) # 'a, b, c'

f"{name!r} is {age} years old"

F-string with repr conversion and expression interpolation

f"{name} scored {score:.1f}"

str.strip()

Remove leading and trailing whitespace from a string

' hello '.strip() # 'hello'

with open('file.txt', 'r') as f: data = f.read()

Open a file and read its entire contents using a context manager

data = open('config.json').read()

with open('file.txt', 'w') as f: f.write(data)

Open a file for writing and write data to it

f.write('Hello\n')

import json json.loads(text)

Parse a JSON string into a Python object

json.loads('{"a": 1}') # {'a': 1}

import csv csv.reader(f)

Read a CSV file row by row using the csv module

for row in csv.reader(f): print(row)

[x*2 for x in lst if x > 0]

List comprehension with transformation and filter condition

[n**2 for n in range(5)] # [0,1,4,9,16]

{k: v for k, v in d.items() if v}

Dict comprehension that filters out falsy values

{k: v for k, v in d.items() if v > 0}

{x for x in lst}

Set comprehension — creates a set of unique values

{w.lower() for w in words}

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

Define a class with an initializer method

p = MyClass(10); print(p.x)

class Dog(Animal): pass

Create a class that inherits from another class

rex = Dog(); rex.speak()

@staticmethod def fn():

Define a static method that doesn't receive the class or instance

MyClass.helper() # no self needed

@classmethod def fn(cls):

Define a class method that receives the class as the first argument

obj = MyClass.from_dict(data)

@property def value(self):

Define a read-only property using the property decorator

print(circle.area) # no parens