Python cheat sheet: strings, lists, dicts and f-strings

This page collects the operations you reach for every day. Strings are immutable, so string methods return a new string and leave the original alone. Lists and dicts are mutable: most of their methods change the object in place and return None. Keeping that difference in mind prevents a whole class of bugs.

f-strings

name, price, ratio, count = "Ada", 1234.5678, 0.256, 7

f"Hi {name}!"          # 'Hi Ada!'
f"{price:.2f}"         # '1234.57'   two decimals
f"{price:,.2f}"        # '1,234.57'  thousands separator
f"{ratio:.1%}"         # '25.6%'     percentage
f"{count:03d}"         # '007'       zero padding
f"[{name:>8}]"         # '[     Ada]' right align in 8 chars
f"[{name:<8}]"         # '[Ada     ]' left align
f"[{name:^8}]"         # '[  Ada   ]' center
f"{255:x} {255:b}"     # 'ff 11111111' hex and binary
f"{count=}"            # 'count=7'   handy for debugging
f"{name!r}"            # "'Ada'"     use repr()

Strings

s = "  Hello, World  "
s.strip()                      # 'Hello, World' (also lstrip / rstrip)
s.lower(), s.upper()           # change case
"hello world".title()          # 'Hello World'
"a,b,,c".split(",")            # ['a', 'b', '', 'c']
"one two  three".split()       # ['one', 'two', 'three'] any whitespace
"-".join(["a", "b", "c"])      # 'a-b-c'
"hello".replace("l", "L")      # 'heLLo'
"hello".startswith("he")       # True (also endswith)
"ell" in "hello"               # True
"hello".find("z")              # -1 (index() raises ValueError instead)
"report.csv".removesuffix(".csv")  # 'report'
"v1.2".removeprefix("v")       # '1.2'
"42".isdigit()                 # True
"7".zfill(3)                   # '007'

Lists

nums = [3, 1, 2]
nums.append(4)            # add one item to the end
nums.extend([5, 6])       # add many items
nums.insert(0, 0)         # insert at a position
nums.pop()                # remove and return the last item
nums.remove(3)            # remove the first matching value
nums.sort()               # sorts in place, returns None
sorted(nums, reverse=True)  # returns a NEW sorted list
nums[-1], nums[1:3], nums[::-1]  # last item, slice, reversed copy
len(nums), sum(nums), min(nums), max(nums)
nums.index(2), nums.count(2)

for i, item in enumerate(["a", "b"], start=1):
    print(i, item)
list(zip(["a", "b"], [1, 2]))    # [('a', 1), ('b', 2)]

Dictionaries

ages = {"ana": 34, "ben": 28}
ages["cy"] = 41                 # add or overwrite
ages.get("zoe")                 # None instead of KeyError
ages.get("zoe", 0)              # 0 as a default
ages.update({"ben": 29})        # merge another dict in
ages.pop("cy")                  # remove and return the value
ages.setdefault("dan", 0)       # set only if the key is missing
"ana" in ages                   # checks keys
merged = ages | {"eve": 50}     # new merged dict

for key, value in ages.items():
    print(key, value)

Comprehensions

A comprehension builds a new collection from an existing one in a single expression. Read it as “give me this for each item, if the condition holds”.

[n * n for n in range(5)]                  # [0, 1, 4, 9, 16]
[n for n in range(10) if n % 2 == 0]       # even numbers
{w: len(w) for w in ["apple", "kiwi"]}     # {'apple': 5, 'kiwi': 4}
{c.lower() for c in "AaBb"}                # set: {'a', 'b'}
sum(n * n for n in range(5))               # generator, no list built

If a comprehension needs more than one if or nested loops, a normal for loop is usually easier to read.

Sorting by a key

sorted(["bob", "Al", "cy"], key=str.lower)        # case-insensitive
sorted(ages.items(), key=lambda kv: kv[1], reverse=True)  # dict by value

Common traps

• result = my_list.sort() sets result to None. Use sorted(my_list) when you need the value.
• s.replace(...) does nothing to s itself; assign the result: s = s.replace(...).
• d["missing"] raises KeyError; use d.get("missing") when a key may be absent.
• removeprefix and removesuffix remove an exact text; strip("xyz") removes any of those characters from both ends.