Files, Errors and Modules

Exceptions

try:
    value = int(text)
except ValueError:
    value = 0
except (TypeError, KeyError) as exc:
    print(exc)
else:
    print("nothing was raised")
finally:
    cleanup()          # always runs

Name what you expect. A bare except: also swallows your own typos and KeyboardInterrupt.

The ones you will meet

Exception	Cause
`ValueError`	right type, wrong value — `int("abc")`
`TypeError`	wrong type — `"a" + 1`
`KeyError`	missing dictionary key
`IndexError`	list index out of range
`AttributeError`	no such attribute, often on `None`
`ZeroDivisionError`	dividing by zero
`FileNotFoundError`	opening something absent
`AssertionError`	a failed `assert`

Raising

raise ValueError("amount must be positive")

class InsufficientFunds(Exception):
    pass

raise InsufficientFunds("not enough")

Return values can be ignored by accident; exceptions cannot.

assert vs raise

	Use for	Note
`assert`	programmer mistakes	removed when Python runs with `-O`
`raise`	bad input from outside	always present

Never validate user input with assert.

Reading a traceback

Traceback (most recent call last):
  File "shop.py", line 22, in <module>
    print(total_price(basket))
  File "shop.py", line 15, in total_price
    return sum(item["price"] for item in items)
KeyError: 'price'

Read the last line — that is the actual error.
Find the last frame in your own code.
The line number is where it surfaced, not always where it started.

Imports

import math                   # math.sqrt(16)
from math import sqrt, pi     # sqrt(16)
import statistics as stats    # stats.mean(...)

Avoid from module import * — it hides where names came from and causes collisions.

Standard library

math

sqrt, floor, ceil, pi, inf, isclose(a, b)

random

random.seed(42)              # makes runs repeatable — essential for testing
random.randint(1, 6)         # inclusive at BOTH ends
random.choice(seq)
random.shuffle(list)         # in place
random.sample(seq, k)        # k distinct items

datetime

from datetime import date, datetime, timedelta

today = date.today()
launch = date(2026, 1, 15)
launch + timedelta(days=30)
(today - launch).days
today.isoformat()            # "2026-01-15"
datetime.now().strftime("%Y-%m-%d %H:%M")

collections

from collections import Counter, defaultdict

Counter("mississippi").most_common(2)   # [('i', 4), ('s', 4)]

totals = defaultdict(int)
totals["a"] += 1              # no need to check whether the key exists

groups = defaultdict(list)
groups["x"].append(1)

JSON

import json

json.dumps(data)                       # object → string
json.loads(text)                       # string → object
json.dump(data, file)                  # object → file
json.load(file)                        # file → object

json.dumps(data, indent=2, sort_keys=True)
json.dumps(data, separators=(",", ":"))   # compact

The s means string. Malformed input raises JSONDecodeError, a subclass of ValueError.

JSON	Python
`null`	`None`
`true` / `false`	`True` / `False`
array	`list` (tuples become lists)
object	`dict` (keys are always strings)

Files

with open("notes.txt", "w") as f:       # "w" TRUNCATES first
    f.write("first line\n")

with open("notes.txt", "a") as f:       # append
    f.write("second\n")

with open("notes.txt") as f:            # "r" is the default
    whole = f.read()

with open("notes.txt") as f:
    for line in f:                      # one at a time — works on huge files
        print(line.rstrip("\n"))

Mode	Meaning
`"r"`	read (default)
`"w"`	write, truncating what is there
`"a"`	append
`"x"`	create, failing if it exists

Always use with — it closes the file even when something raises.

A safe reading pattern

def count_lines(path):
    try:
        with open(path) as f:
            return sum(1 for line in f if line.strip())
    except FileNotFoundError:
        return 0

This cheatsheet is the summary. If you want to build it yourself, the Files, Errors and Modules course walks you through it in the browser — the first lesson is free.