Python and Libraries Cheatsheet

Main

if __name__ == '__main__':
    main()

List

<list>[<inclusive from>:<exclusive to>:<step size>]
<list>.extend(<list>)
<list>.sort()
<list>.reverse()
sum(<list>)
sorted_by_second = sorted(<list>, key=lambda tup: tup[1])

Dictionary

<dict>.items()
<dict>.get(<key>, <default>)
<dict>.setdefault(<key>, <default>)
<dict>.update(<dict>)
collections.defaultdict(<type>)  # Creates list 

Init from two lists

dict(zip(keys, values))

Set

<set> = set()
<set>.add(<el>)
<set>.update(<set>)
<set>.union(<set>)
<set>.intersection(<set>)
<set>.difference(<set>)

Range

range(<to exclusive>)
range(<from inclusive>, <to exclusive>)
range(<from inclusive>, <to exclusive>, <step size>)  # Negative step for backward

Enumerate

for i, <el> in enumerate(<list>)

Named Tuples

>>> TestResults = collections.namedtuple('TestResults', ['filed', 'attempted'])
>>> TestResults(1, 2)
TestResults(filed=1, attempted=2)

Iterator

Reads input until it reaches empty line.

for line in iter(input, ''):
    print(line)

Use partial from functools if function needs arguments.

Skips first element.

next(<iter>)
for element in <iter>:
    ...

Type

type(<el>) is int/str/set/list

import numbers
isinstance(<el>, numbers.Number)

String

str.replace(<text>, <old>, <new>)
<str>.isnumeric()
<str>.split()
<str>.strip()
'<str>'.join(<list>)

Print

print(<el1>, <el2>, end='', sep='', file=<file>)

Regex

import re
re.sub(<regex>, <new>, <text>)
re.search(<regex>, <text>)

Format

'{}'.format(<el>)

{:<min width>}  -> '<el>    '
{:><min width>} -> '    <el>'
{:^<min width>} -> '  <el>  '
{:_<min width>}  -> '<el>____'
{:.<max width>} -> '<e>'
{:<max widht>.<min width>} -> '    <e>'
{:<max width>.<no of decimals>f} -> '  3.14'

>>> person = {'name': 'Jean-Luc', 'height': 187.1}
>>> '{p[height]:.0f}'.format(p=person)
'187'

Text Wrap

import textwrap
textwrap.wrap(<text>, <width>)

Random

import random
random.random()
random.randint(<from inclusive>, <to inclusive>)
random.shuffle(<list>)

Infinity

float("inf")

Datetime

import datetime
now = datetime.datetime.now()
now.strftime('%Y%m%d')
now.strftime('%Y%m%d%H%M%S')

Args

args = (1, 2)
kwargs = {'x': 3, 'y': 4, 'z': 5}
func(*args, **kwargs)
# same as
func(1, 2, x=3, y=4, z=5)

Inline

Lambda

lambda <arg1>, <arg2>: <return value>
lambda: <return value>

Comprehension

[i+1 for i in range(10)]
[i for i in range(10) if i>5]
[i+j for i in range(10) for j in range(10)]
{i: i*2 for i in range(10)}
(x+5 for x in range(0, 10)) - generator!

Map, Filter, Reduce

map(lambda x: x+1, range(10))
filter(lambda x: x>5, range(10))
functools.reduce(combining_function, list_of_inputs)

Closure

def mult_clos(x):
   def wrapped(y):
            return x * y
    return wrapped 

mul_by_3 = mult_clos(3)

or

from functools import partial
partial(<function>, <parameter>)

Decorator

@closure_name
def function_that_gets_passed_to_closure():
    ...

Debugger example

from functools import wraps

def debug(func):
    @wraps(func)  # Nedded for metadata copying (func name, etc)
    def wrapper(*args, **kwargs):
        print(func.__name__)
        return func(*args, **kwargs)
    return wrapper

@debug
def add(x, y):
    return x + y

Generator

def step(start, step):
    while True:
        yield start
        start += step

stepper = step(10, 2)
next(stepper)

Class

Class

class <name>:
    def __init__(self, <arg>):
        self.a = <arg>
    def __repr__(self):
        return str({'a': self.a})
    def __str__(self):
        return str(self.a)

Enum

import enum
class <name>(enum.Enum):
    <value> = <index>  # or auto()

Copy

import copy
copy.copy(<object>)
copy.deepcopy(<object>)

System

Arguments

import sys
sys.argv

Read File

with open(<filename>, encoding='utf-8') as file:
    return file.readlines()

Write to File

with open(<filename>, 'w', enconding='utf-8') as file:
    file.write(<text>)

Execute Command

import os
os.popen(<command>).read()

Input

filename = input('Enter a file name: ')

Print lines until EOF

while True:
    try:
        print(input())
    except EOFError:
        break

JSON

import json

Read File

with open(<filename>, encoding='utf-8') as file:
    return json.load(file)

Write to File

with open(<filename>, 'w', enconding='utf-8') as file:
    file.write(json.dumps(<object>))

SQLite

import sqlite3
db = sqlite3.connect(<filename>)

Read

cursor = db.execute(<query>)
if cursor:
    cursor.<fetchone/fetchall>()
db.close()

Write

db.execute(<query>)
db.commit()

Exceptions

while True:
    try:
        x = int(input("Please enter a number: "))
        break
    except ValueError:
        print("Oops!  That was no valid number.  Try again...")

Threading

import threading

Thread

thread = threading.Thread(target=<function>, args=(<first arg>, ))
thread.start()
thread.join()

Lock

lock = threading.Rlock()
lock.acquire()
lock.release()

Itertools

Every function returns an generator and can accept any collection. All examples should be passed to list() to get the output.

from itertools import *

Chain

>>> chain([1,2], range(3,5))
[1, 2, 3, 4]

Combinations

>>> combinations("abc", 2)
[('a', 'b'), ('a', 'c'), ('b', 'c')]

Permutations

>>> permutations("abc", 2)
[('a', 'b'), ('a', 'c'), ('b', 'a'), ('b', 'c'), ('c', 'a'), ('c', 'b')]

Compress

>>> compress("abc", [True, 0, 23])
['a', 'c']

Count

>>> a = count(5, 2)
>>> next(a), next(a)
(5, 7)

Cycle

>>> a = cycle("abc")
>>> [next(a) for _ in range(10)]
['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c', 'a']

Groupby

>>> {k: list(v) for k, v in groupby("aabbbc")}
{'a': ['a', 'a'], 'b': ['b', 'b', 'b'], 'c': ['c']}

>>> a = [{"id": 1, "name": "bob"}, {"id": 2, "name": "bob"}, {"id": 3, "name": "peter"}]
>>> {k: list(v) for k, v in groupby(a, key=lambda x: x["name"])}
{'bob': [{'id': 1, 'name': 'bob'}, {'id': 2, 'name': 'bob'}], 'peter': [{'id': 3, 'name': 'peter'}]}

Product

>>> list(product('ab', [1,2]))
[('a', 1), ('a', 2), ('b', 1), ('b', 2)]

ifilter/imap/izip

Filter, map and zip functions that return generators instead of iterators

Introspection and Metaprograming

Inspecting code at runetime and code that generates code.

>>> class B:
...     def __init__(self):
...             self.a= 'sdfsd'
...             self.b = 123324
>>> b = B()

Getattr

>>> getattr(b, 'a')
'sdfsd'

same as

B.__getattribute__(b, 'a')

Hasattr

>>> hasattr(b, 'c')
False

Setattr

>>> setattr(b, 'c', 10)

Type

Type is the root class. If only passed the object it returns it's type. Otherwise it creates new class (and not the instance!).

type(class_name, parents[tuple], attributes[dict])

BB = type('B', (), {'a': 'sdfsd', 'b': 123324}
b = BB()

MetaClass

Classes that creates classes.

def my_meta_class(name, parents, attrs):
    ... do stuff
    return type(name, parents, attrs)

or

class MyMetaClass(type):
    def __new__(klass, name, parents, attrs):
        ... do stuff
        return type.__new__(klass, name, parents, attrs)

Do Stuff

• Look at the attributes
• Set new attributes
• Create functions dynamically
• Traverse the parent classes
• Change values in the class

Metaclass Attr

When class is created it checks if it has metaclass defined. If not, it recursively checks if any of his parents has it defined, and eventually comes to type.

class BlaBla:
    __metaclass__ = Bla