Code & computing / QUICK REFERENCE

Python essentials

Built-in types, comprehensions, iteration, functions and common standard-library patterns.

36 entries

Values and strings

Length: len('atlas')
Returns 5; counts code points, not displayed grapheme clusters. [1][6]
Integer conversion: int('42')
Invalid numeric text raises ValueError. [1][6]
Floating-point division: 7 / 2
Returns 3.5. [1]
Floor division: 7 // 2
Returns 3; negative results round downward. [1]
Remainder: 7 % 2
Returns 1. [1]
Formatted string: name = 'Atlas'; message = f'Hello, {name}'
Substitutes the variable into a new string. [1]
Strip surrounding whitespace: ' atlas '.strip()
Does not change the original immutable string. [1]
Split words: 'one two'.split()
Splits on runs of whitespace. [1]
Join strings: ', '.join(['one', 'two'])
Every joined element must be a string. [1]
Slice a string: 'abcdef'[1:4]
Returns bcd; the stop index is excluded. [1]

Create a list: items = [1, 2, 3]
Lists preserve order and allow duplicates. [2]
Last element: items[-1]
Raises IndexError for an empty list. [2]
Append one element: items.append(4)
Mutates the list and returns None. [2]
Sorted copy: sorted([3, 1, 2])
Returns a new list without changing the input. [2][6]
Comprehension: [x * x for x in range(5)]
Produces squares for 0 through 4. [2][6]
Create a dictionary: record = {'name': 'Atlas', 'count': 3}
Keys map to values. [2]
Lookup with fallback: record.get('missing', 0)
Uses the fallback only when the key is absent. [2]
Dictionary entries: list(record.items())
Returns key-value pairs in insertion order. [2]
Unique values: set([1, 1, 2])
A set removes duplicates; do not rely on display order. [2]
Membership: 'name' in record
Tests dictionary keys, not values. [2]

Index while iterating: for i, value in enumerate(['a', 'b']): print(i, value)
Indexes start at zero unless a start value is given. [3][6]
Pair two iterables: list(zip([1, 2], ['a', 'b']))
Default zip stops at the shorter iterable. [3][6]
Half-open range: list(range(2, 8, 2))
Produces 2, 4 and 6. [3][6]
Small function: def square(x): return x * x
Indentation defines the function body. [3]
Conditional expression: label = 'even' if 4 % 2 == 0 else 'odd'
Selects one expression's value. [3]
Truth test: bool([])
Empty containers are false. [3][6]
Identity check for None: value is None
Prefer identity rather than == for this singleton. [3]
Generator expression: sum(x * x for x in range(5))
Accumulates without creating a list of squares. [3][6]

Open text explicitly: with open('notes.txt', encoding='utf-8') as f: text = f.read()
The context manager closes the file. [4]
Parse JSON: import json; record = json.loads('{"count": 3}')
Parses data; do not use eval for JSON. [7]
Serialize JSON: json.dumps({'count': 3})
Import json first; custom objects need handling. [7]
Build a filesystem path: from pathlib import Path; path = Path('data') / 'notes.txt'
Avoids manual separator concatenation. [8]
Count values: from collections import Counter; counts = Counter('banana')
Produces frequencies by distinct element. [9]
Current UTC time: from datetime import datetime, timezone; now = datetime.now(timezone.utc)
Creates an aware datetime rather than a naive one. [10]
Catch a conversion error: try: number = int('bad') except ValueError: number = None
Handles the expected exception, not every error indiscriminately. [5][6]
Enumerate from one: list(enumerate(['a', 'b'], start=1))
Useful for human-readable row numbering. [5][6]

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Updated 2026-10-19Python 3 · portable core syntax

cheatsheetatlas.com/sheets/python/ · Updated 2026-10-19 · Python 3 · portable core syntax