Python Quick Reference

A printable, at-a-glance reference for Python 3. Covers the essentials — perfect for coding interviews, exam prep, or daily use.

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Syntax Basics

# Single-line comment
""" Multi-line
    comment """

# Variable assignment (no type declaration needed)
x = 5          # int
y = 3.14       # float
name = "Alice" # str

# Printing
print("Hello")            # with newline
print("No newline", end="")  # without newline
print(f"Value is {x}")    # f-string interpolation

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Data Types

Type	Example	Mutable	Notes
int	x = 42	No	Arbitrary precision
float	x = 3.14	No	Double-precision
str	s = "hello"	No	Immutable sequence of chars
bool	b = True	No	Subclass of int
NoneType	x = None	No	Represents absence of value
list	lst = [1, 2, 3]	Yes	Ordered, indexed, allows duplicates
tuple	t = (1, 2, 3)	No	Ordered, immutable
dict	d = {"a": 1}	Yes	Key-value pairs (3.7+ preserves order)
set	s = {1, 2, 3}	Yes	Unordered, no duplicates

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Operators

Arithmetic

Operator	Description	Example
+	Addition	5 + 3 → 8
-	Subtraction	5 - 3 → 2
*	Multiplication	5 * 3 → 15
/	Division (float)	5 / 2 → 2.5
//	Floor division	5 // 2 → 2
%	Modulus	5 % 2 → 1
**	Exponentiation	5 ** 2 → 25

Comparison

Operator	Description	Example
==	Equal to	5 == 5 → True
!=	Not equal to	5 != 3 → True
>	Greater than	5 > 3 → True
<	Less than	5 < 3 → False
>=	Greater than or equal	5 >= 5 → True
<=	Less than or equal	5 <= 3 → False

Logical

Operator	Description	Example
and	Both True	True and False → False
or	At least one True	True or False → True
not	Inverts truth value	not True → False

Assignment

Operator	Example	Equivalent to
=	x = 5	—
+=	x += 3	x = x + 3
-=	x -= 3	x = x - 3
*=	x *= 3	x = x * 3
/=	x /= 3	x = x / 3
//=	x //= 3	x = x // 3
%=	x %= 3	x = x % 3
**=	x **= 3	x = x ** 3

Membership & Identity

Operator	Description	Example
in	Is element in sequence?	"a" in "cat" → True
not in	Is element not in sequence?	"z" not in "cat" → True
is	Same object identity?	x is None
is not	Different object identity?	x is not None

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String Methods

Method	Description	Example
.upper()	Convert to uppercase	"hi".upper() → "HI"
.lower()	Convert to lowercase	"HI".lower() → "hi"
.split(sep)	Split by delimiter into list	"a,b,c".split(",") → ["a","b","c"]
.join(iterable)	Join elements with string	",".join(["a","b"]) → "a,b"
.replace(old, new)	Replace occurrences	"hi".replace("i","ello") → "hello"
.strip()	Remove leading/trailing whitespace	" hi ".strip() → "hi"
.find(sub)	Index of first occurrence	"hello".find("l") → 2
.startswith(prefix)	Check prefix	"hi".startswith("h") → True
.endswith(suffix)	Check suffix	"hi".endswith("i") → True
.count(sub)	Count non-overlapping occurrences	"lol".count("l") → 2

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List Methods

Method	Description	Example
.append(x)	Add x to end	[1].append(2) → [1, 2]
.extend(iter)	Append all from iterable	[1].extend([2,3]) → [1,2,3]
.insert(i, x)	Insert x at index i	[1,3].insert(1,2) → [1,2,3]
.remove(x)	Remove first occurrence of x	[1,2,1].remove(1) → [2,1]
.pop(i)	Remove & return item at i (default last)	[1,2,3].pop() → 3
.sort()	Sort in-place	[3,1,2].sort() → [1,2,3]
.reverse()	Reverse in-place	[1,2,3].reverse() → [3,2,1]
.index(x)	Index of first occurrence	[1,2,3].index(2) → 1
.count(x)	Count occurrences	[1,2,2].count(2) → 2
.copy()	Shallow copy	lst.copy()

List slicing: lst[start:stop:step] — [1,2,3,4][1:3] → [2,3]

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Dictionary Methods

Method	Description	Example
.keys()	View of all keys	d.keys()
.values()	View of all values	d.values()
.items()	View of (key, value) pairs	d.items()
.get(k, default)	Safe lookup	d.get("x", 0) → 0
.update(d2)	Merge from another dict	d.update({"b": 2})
.pop(k, default)	Remove & return value	d.pop("a")
.setdefault(k, v)	Set default if key missing	d.setdefault("a", 0)
.clear()	Remove all items	d.clear()

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Control Flow

if-elif-else

if condition:
    # do something
elif other_condition:
    # do something else
else:
    # fallback

for Loop

for item in iterable:
    # process item

for i in range(5):           # 0, 1, 2, 3, 4
for i in range(2, 6):        # 2, 3, 4, 5
for i in range(1, 10, 2):    # 1, 3, 5, 7, 9

while Loop

while condition:
    # repeat while condition is True

Loop Control

break      # exit loop immediately
continue   # skip to next iteration
else:      # runs if loop didn't break

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Functions

def

def function_name(param1, param2="default"):
    """Docstring"""
    result = param1 + param2
    return result

*args / **kwargs

def func(*args, **kwargs):
    # args  → tuple of positional arguments
    # kwargs → dict of keyword arguments

lambda

lambda x: x * 2           # single expression, implicit return
lambda a, b: a + b

# Usage
sorted(list, key=lambda x: x[1])
map(lambda x: x * 2, [1, 2, 3])

return

return value       # exit and return value
return             # exit and return None
# No return → function returns None

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List Comprehension

# Basic
[x * 2 for x in range(5)]           # → [0, 2, 4, 6, 8]

# With condition
[x for x in range(10) if x % 2 == 0] # → [0, 2, 4, 6, 8]

# Nested
[(x, y) for x in [1,2] for y in [3,4]]

# Dict comprehension
{k: v for k, v in zip(keys, values)}

# Set comprehension
{x**2 for x in [1, 2, 2, 3]}  # → {1, 4, 9}

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Classes & OOP

class Parent:
    def __init__(self, name):
        self.name = name          # instance attribute

    def greet(self):              # instance method
        return f"Hi, {self.name}"

class Child(Parent):              # inheritance
    def __init__(self, name, age):
        super().__init__(name)    # call parent constructor
        self.age = age

    def greet(self):              # method override
        return f"Hey, {self.name} ({self.age})"

# Usage
obj = Child("Alice", 10)
print(obj.greet())  # Hey, Alice (10)

@property

class Circle:
    def __init__(self, radius):
        self._radius = radius

    @property
    def area(self):
        return 3.14 * self._radius ** 2

    @property
    def radius(self):
        return self._radius

    @radius.setter
    def radius(self, value):
        if value < 0:
            raise ValueError("Radius cannot be negative")
        self._radius = value

Access Modifiers (Convention)

self.public       # accessible everywhere
self._protected   # "internal use" (convention)
self.__private    # name-mangled to _ClassName__private

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File I/O

# Read entire file
with open("file.txt", "r") as f:
    content = f.read()

# Read line by line
with open("file.txt", "r") as f:
    for line in f:
        print(line.strip())

# Write (overwrite)
with open("file.txt", "w") as f:
    f.write("Hello, World!")

# Append
with open("file.txt", "a") as f:
    f.write("\nMore text")

# Read all lines into list
with open("file.txt", "r") as f:
    lines = f.readlines()

File Modes

Mode	Description
"r"	Read (default)
"w"	Write (overwrites)
"a"	Append
"x"	Exclusive creation (fails if exists)
"r+"	Read + write
"b"	Binary mode (e.g., "rb", "wb")

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Exception Handling

try:
    # code that may raise an exception
    result = risky_operation()
except ValueError as e:
    # handle specific exception
    print(f"Value error: {e}")
except (TypeError, ZeroDivisionError) as e:
    # handle multiple exception types
    print(f"Error: {e}")
except Exception as e:
    # catch-all (use sparingly)
    print(f"Unexpected error: {e}")
else:
    # runs if no exception was raised
    print("Success!")
finally:
    # always runs (cleanup)
    cleanup()

# Manually raising
raise ValueError("Invalid value")
raise  # re-raise current exception

Common Exception Types

Exception	Raised When
ValueError	Function receives invalid argument value
TypeError	Operation on inappropriate type
IndexError	Sequence subscript out of range
KeyError	Dictionary key not found
FileNotFoundError	File does not exist
ZeroDivisionError	Division by zero
AttributeError	Attribute reference fails
ImportError	Import fails
StopIteration	Iterator exhausted

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Common Built-in Functions

Function	Description	Example
print(x)	Print to console	print("hi")
len(x)	Length of sequence	len([1,2,3]) → 3
type(x)	Type of object	type(5) → <class 'int'>
range(n)	Generate integer sequence	list(range(3)) → [0,1,2]
input(prompt)	Read user input as string	name = input("Name: ")
int(x)	Convert to integer	int("5") → 5
str(x)	Convert to string	str(5) → "5"
float(x)	Convert to float	float("3.14") → 3.14
list(x)	Convert to list	list("abc") → ["a","b","c"]
dict(x)	Convert to dict	dict([("a",1)]) → {"a":1}
set(x)	Convert to set	set([1,2,2]) → {1,2}
sorted(x)	Return sorted list	sorted([3,1,2]) → [1,2,3]
enumerate(x)	Indexed pairs	list(enumerate(["a","b"])) → [(0,"a"),(1,"b")]
zip(a, b)	Parallel iteration	list(zip([1,2], ["a","b"])) → [(1,"a"),(2,"b")]
map(f, x)	Apply function to each	list(map(str, [1,2])) → ["1","2"]
filter(f, x)	Filter by predicate	list(filter(None, [0,1,0])) → [1]
abs(x)	Absolute value	abs(-5) → 5
sum(x)	Sum of iterable	sum([1,2,3]) → 6
min(x)	Minimum value	min([3,1,2]) → 1
max(x)	Maximum value	max([3,1,2]) → 3
round(x, n)	Round to n decimal places	round(3.14159, 2) → 3.14
any(x)	Any element truthy?	any([False, True]) → True
all(x)	All elements truthy?	all([True, False]) → False

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Module Imports

import math                    # import entire module
import math as m              # with alias
from math import sqrt          # import specific name
from math import *             # import all (avoid)
from math import sqrt as s    # with alias

# Common standard library modules
import os      # operating system interface
import sys     # system-specific parameters
import json    # JSON encoder/decoder
import csv     # CSV file reading/writing
import re      # regular expressions
import datetime  # date and time
import random  # random number generation
import math    # mathematical functions
import statistics  # statistical functions

__name__ Guard

if __name__ == "__main__":
    # runs only when script is executed directly
    main()