Python Cheatsheet
Data Types
Core Python types and how to inspect them.
| Syntax | Description |
|---|---|
42, 3.14, -7 |
int, float literals |
True, False |
bool |
"hello", 'world' |
str (immutable) |
[1, 2, 3] |
list — mutable, ordered |
(1, 2, 3) |
tuple — immutable, ordered |
{"a": 1, "b": 2} |
dict — key-value pairs |
{1, 2, 3} |
set — unique, unordered |
None |
Absence of a value |
type(x) |
Get type of x |
isinstance(x, int) |
Check if x is of a given type |
Operators
Arithmetic, comparison, logical, and membership operators.
| Operator | Description |
|---|---|
+, -, *, / |
Add, subtract, multiply, divide |
// |
Floor division |
% |
Modulo (remainder) |
** |
Exponentiation |
==, != |
Equal, not equal |
<, >, <=, >= |
Comparison |
and, or, not |
Logical operators |
in, not in |
Membership test |
is, is not |
Identity test |
x if cond else y |
Ternary (conditional) expression |
String Methods
Common operations on str objects. Strings are immutable — methods return a new string.
| Method | Description |
|---|---|
s.upper() / s.lower() |
Convert to upper / lowercase |
s.strip() |
Remove leading and trailing whitespace |
s.lstrip() / s.rstrip() |
Remove leading / trailing whitespace |
s.split(",") |
Split on delimiter, return list |
",".join(lst) |
Join list elements into a string |
s.replace("old", "new") |
Replace all occurrences |
s.find("x") |
First index of substring (-1 if not found) |
s.startswith("x") |
True if string starts with prefix |
s.endswith("x") |
True if string ends with suffix |
s.count("x") |
Count non-overlapping occurrences |
s.isdigit() / s.isalpha() |
Check all digits / all letters |
len(s) |
String length |
s[i], s[i:j], s[::step] |
Index, slice, step |
"x" in s |
Substring membership test |
String Formatting
Embed values in strings.
| Syntax | Description |
|---|---|
f"Hello, {name}" |
f-string (Python 3.6+) |
f"{val:.2f}" |
Float with 2 decimal places |
f"{val:>10}" |
Right-align in 10 characters |
f"{val:,}" |
Thousands separator |
f"{{name}}" |
Literal braces in output |
"Hello, {}".format(name) |
str.format() |
"Hello, %s" % name |
%-formatting (legacy) |
List Methods
Common operations on list objects.
| Method | Description |
|---|---|
lst.append(x) |
Add element to end |
lst.extend([x, y]) |
Add multiple elements to end |
lst.insert(i, x) |
Insert element at index i |
lst.remove(x) |
Remove first occurrence of x |
lst.pop() |
Remove and return last element |
lst.pop(i) |
Remove and return element at index i |
lst.index(x) |
First index of x |
lst.count(x) |
Count occurrences of x |
lst.sort() |
Sort in place (ascending) |
lst.sort(reverse=True) |
Sort in place (descending) |
lst.reverse() |
Reverse in place |
lst.copy() |
Shallow copy |
lst.clear() |
Remove all elements |
len(lst) |
List length |
lst[i], lst[i:j] |
Index and slice |
x in lst |
Membership test |
Dictionary Methods
Common operations on dict objects.
| Method | Description |
|---|---|
d[key] |
Get value by key (raises KeyError if missing) |
d.get(key) |
Get value or None if key not found |
d.get(key, default) |
Get value or default if key not found |
d[key] = val |
Set or update a value |
del d[key] |
Delete a key |
d.pop(key) |
Remove key and return its value |
d.keys() |
View of all keys |
d.values() |
View of all values |
d.items() |
View of all (key, value) pairs |
d.update(d2) |
Merge another dict into d |
d.setdefault(key, val) |
Set key if missing, return value |
key in d |
Check if key exists |
len(d) |
Number of key-value pairs |
{**d1, **d2} |
Merge dicts (Python 3.9+: d1 | d2) |
Sets
Common operations on set objects.
| Syntax | Description |
|---|---|
set() |
Empty set (use this, not {}) |
s.add(x) |
Add element |
s.remove(x) |
Remove element (raises KeyError if missing) |
s.discard(x) |
Remove element (no error if missing) |
s1 | s2 |
Union |
s1 & s2 |
Intersection |
s1 - s2 |
Difference |
s1 ^ s2 |
Symmetric difference |
s1 <= s2 |
Subset check |
x in s |
Membership test |
Control Flow
Conditionals, loops, and flow control keywords.
| Syntax | Description |
|---|---|
if cond: / elif cond: / else: |
Conditional blocks |
for item in iterable: |
Iterate over a sequence |
for i, v in enumerate(lst): |
Iterate with index and value |
while cond: |
Loop while condition is true |
break |
Exit loop immediately |
continue |
Skip to next iteration |
pass |
No-op placeholder |
match x: / case val: |
Pattern matching (Python 3.10+) |
Functions
Define and call functions.
| Syntax | Description |
|---|---|
def func(x, y): |
Define a function |
return val |
Return a value |
def func(x=0): |
Default argument |
def func(*args): |
Variable positional arguments |
def func(**kwargs): |
Variable keyword arguments |
lambda x: x * 2 |
Anonymous (lambda) function |
func(y=1, x=2) |
Call with keyword arguments |
result = func(x) |
Call and capture return value |
File I/O
Open, read, and write files.
| Syntax | Description |
|---|---|
open("f.txt", "r") |
Open for reading (default mode) |
open("f.txt", "w") |
Open for writing — creates or overwrites |
open("f.txt", "a") |
Open for appending |
open("f.txt", "rb") |
Open for reading in binary mode |
with open("f.txt") as f: |
Open with context manager (auto-close) |
f.read() |
Read entire file as a string |
f.read(n) |
Read n characters |
f.readline() |
Read one line |
f.readlines() |
Read all lines into a list |
for line in f: |
Iterate over lines |
f.write("text") |
Write string to file |
f.writelines(lst) |
Write list of strings |
Exception Handling
Catch and handle runtime errors.
| Syntax | Description |
|---|---|
try: |
Start guarded block |
except ValueError: |
Catch a specific exception |
except (TypeError, ValueError): |
Catch multiple exception types |
except Exception as e: |
Catch and bind exception to variable |
else: |
Run if no exception was raised |
finally: |
Always run — use for cleanup |
raise ValueError("msg") |
Raise an exception |
raise |
Re-raise the current exception |
Comprehensions
Build lists, dicts, sets, and generators in one expression.
| Syntax | Description |
|---|---|
[x for x in lst] |
List comprehension |
[x for x in lst if cond] |
Filtered list comprehension |
[f(x) for x in range(n)] |
Comprehension with expression |
{k: v for k, v in d.items()} |
Dict comprehension |
{x for x in lst} |
Set comprehension |
(x for x in lst) |
Generator expression (lazy) |
Useful Built-ins
Frequently used Python built-in functions.
| Function | Description |
|---|---|
print(x) |
Print to stdout |
input("prompt") |
Read user input as string |
len(x) |
Length of sequence or collection |
range(n) / range(a, b, step) |
Integer sequence |
enumerate(lst) |
(index, value) pairs |
zip(a, b) |
Pair elements from two iterables |
map(func, lst) |
Apply function to each element |
filter(func, lst) |
Filter elements by predicate |
sorted(lst) |
Return sorted copy |
sum(lst), min(lst), max(lst) |
Sum, minimum, maximum |
abs(x), round(x, n) |
Absolute value, round to n places |
int(x), float(x), str(x) |
Type conversion |
Related Guides
| Guide | Description |
|---|---|
| Python f-Strings | String formatting with f-strings |
| Python Lists | List operations and methods |
| Python Dictionaries | Dictionary operations and methods |
| Python for Loop | Iterating with for loops |
| Python while Loop | Looping with while |
| Python if/else Statement | Conditionals and branching |
| How to Replace a String in Python | String replacement methods |
| How to Split a String in Python | Splitting strings into lists |
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