Python reference

Types and conversion

int('42')

text to whole number

float('3.14')

text to decimal number

str(42)

anything to text

bool(x)

truthiness — 0, '', [], {}, None are False

type(x)

what type is this?

isinstance(x, int)

type check that respects subclasses

round(3.14159, 2)

3.14

Operators

+ - *

add, subtract, multiply

/

true division — always a float

//

floor division — drops the remainder

%

remainder (modulo)

**

exponent

+= -= *= //=

update in place

== != < > <= >=

comparison, yields a bool

and or not

combine conditions

in not in

membership

is is not

same object (use == for equality)

Strings

Immutable — every method returns a new string.

f'{name} is {age}'

f-string interpolation

f'{value:.2f}'

two decimal places

s.upper() / s.lower()

change case

s.strip()

remove surrounding whitespace

s.title() / s.capitalize()

title case / first letter only

s.replace(a, b)

substitute

s.split(',')

text to list

','.join(items)

list to text

s.startswith(x) / s.endswith(x)

prefix / suffix test

s[0] s[-1] s[1:4] s[::-1]

index, slice, reverse

len(s)

character count

Lists

Ordered and mutable.

items.append(x)

add one item to the end

items.extend(other)

add each item of another list

items.insert(i, x)

add at a position

items.pop() / items.pop(i)

remove and return

items.remove(x)

remove first matching value

items.sort() / items.sort(reverse=True)

in place, returns None

sorted(items)

new sorted list

items.copy() / items[:]

independent copy

len / sum / min / max

aggregate built-ins

[f(x) for x in items if cond]

comprehension

Dictionaries and sets

d['key']

read — raises KeyError if absent

d.get('key', default)

read safely

d['key'] = value

add or update

del d['key'] / d.pop('key', None)

remove

d.keys() / d.values() / d.items()

views for looping

for k, v in d.items():

the loop you want most often

'key' in d

tests keys, not values

{k: v for k, v in pairs}

dict comprehension

set(items)

unique values

a | b a & b a - b

union, intersection, difference

Control flow

if cond: elif cond: else:

branch — first match wins

for x in collection:

once per item

for i in range(start, stop, step)

stop is excluded

for i, x in enumerate(items, start=1)

index and value

for a, b in zip(xs, ys)

walk two sequences together

while cond:

repeat until the condition fails

break / continue

exit the loop / skip this pass

pass

do nothing (placeholder)

Functions and classes

def f(a, b=1):

definition with a default

return value

hand a result back to the caller

return a, b

return several values as a tuple

f(*args, **kwargs)

variable positional / keyword arguments

lambda x: x * 2

small anonymous function

class Dog:

define a type

def __init__(self, ...)

runs on creation

def __str__(self)

what print() shows

class Puppy(Dog):

inherit

super().__init__(...)

run the parent's initialiser

Errors

try: / except ValueError as e:

handle a specific failure

else:

runs only if nothing was raised

finally:

always runs — cleanup goes here

raise ValueError('message')

reject bad input

SyntaxError

typo: missing colon, bracket, or quote

NameError

variable does not exist

TypeError

wrong type, e.g. 'a' + 1

ValueError

right type, impossible value, e.g. int('x')

IndexError / KeyError

no such position / no such key

ZeroDivisionError

divided by zero

Files and modules

with open(p) as f:

read; closes automatically

open(p, 'w')

write — WIPES the existing file

open(p, 'a')

append — keeps existing content

f.read() / f.readlines()

whole file / list of lines

for line in f:

stream one line at a time

import math

then math.sqrt(9)

from math import sqrt

then sqrt(9)

import numpy as np

import under a shorter name

if __name__ == '__main__':

run only when executed directly

Useful standard library

math

sqrt, floor, ceil, pi, factorial

random

randint, choice, shuffle, sample, seed

datetime

date, datetime, timedelta, strftime

json

dumps (to text), loads (from text)

collections

Counter, defaultdict, deque

statistics

mean, median, mode, stdev

os / pathlib

paths and the filesystem

re

regular expressions