Python Cheat Sheet

• pip install <package_name> — Install a package
• pip install <package_name>==<version> — Install a specific version
• pip install -r requirements.txt — Install from a requirements file
• pip uninstall <package_name> — Uninstall a package
• pip list — List installed packages
• pip freeze > requirements.txt — Generate requirements.txt from current env

• python -m venv <env_name> — Create a new virtual environment
• Windows (Activate): .\<env_name>\Scripts\activate
• macOS/Linux (Activate): source <env_name>/bin/activate
• deactivate — Deactivate the current virtual environment

Basic (sys.argv)

• import sys
• script_name = sys.argv[0]
• first_arg = sys.argv[1]
• all_args = sys.argv[1:] — list of arguments

Advanced (argparse)

import argparse

parser = argparse.ArgumentParser(description="My script.")
parser.add_argument('file', help='Input file path')         # positional
parser.add_argument('--count', type=int, default=1)         # optional
parser.add_argument('-v', '--verbose', action='store_true') # flag
args = parser.parse_args()

print(args.file)
print(args.count)
if args.verbose: print("Verbose mode on.")

Comments

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

Variables & Data Types

• name = "Alice" — String
• age = 30 — Integer
• height = 1.75 — Float
• is_student = True — Boolean

Operators

• + - * / % — Arithmetic
• == != < > <= >= — Comparison
• and or not — Logical

Input / Output

• print("Hello, World!")
• user_input = input("Enter your name: ")

Type Conversion

• int("123")  ·  str(45)  ·  float("3.14")

Lists — Mutable, Ordered

• my_list = [1, 2, 3, "a"]
• my_list.append(4)
• my_list[0] — access element  ·  my_list[1:3] — slice
• len(my_list)

Tuples — Immutable, Ordered

• my_tuple = (1, 2, "a")  ·  my_tuple[0]

Dictionaries — Key-Value Pairs

• my_dict = {"name": "Bob", "age": 25}
• my_dict["name"]  ·  my_dict["city"] = "NY"
• my_dict.keys()  ·  my_dict.values()  ·  my_dict.items()

Sets — Unordered, Unique Elements

• my_set = {1, 2, 3, 2} — results in {1, 2, 3}
• my_set.add(4)  ·  my_set.remove(1)

if / elif / else

if condition1:
    # code
elif condition2:
    # code
else:
    # code

for loops

for item in iterable:
    # code
for i in range(5):   # 0, 1, 2, 3, 4
    # code

while loops

while condition:
    # code
    # remember to change condition to avoid infinite loop

• break — Exit a loop
• continue — Skip current iteration, move to next

def greet(name):
    return f"Hello, {name}!"

def add(a, b=0):   # default parameter
    return a + b

• message = greet("Alice")
• sum_val = add(5)  ·  sum_val = add(5, 3)

class Dog:
    def __init__(self, name, breed):
        self.name = name
        self.breed = breed

    def bark(self):
        return f"{self.name} says Woof!"

• my_dog = Dog("Buddy", "Golden Retriever")
• print(my_dog.name)  ·  print(my_dog.bark())

Reading

with open('myfile.txt', 'r') as f:
    content = f.read()
    # or: for line in f: print(line.strip())

Writing (overwrites)

with open('myfile.txt', 'w') as f:
    f.write("Hello, file!\n")

Appending

with open('myfile.txt', 'a') as f:
    f.write("Appended line.\n")

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Cannot divide by zero!")
except ValueError as e:
    print(f"Value Error: {e}")
except Exception as e:
    print(f"Unexpected error: {e}")
else:
    print("No errors occurred.")
finally:
    print("This always runs.")

• squares = [x*x for x in range(5)] → [0, 1, 4, 9, 16]
• evens = [x for x in range(10) if x % 2 == 0] → [0, 2, 4, 6, 8]

• f-strings (3.6+): f"Name: {name}, Age: {age}"
• .format(): "Name: {}, Age: {}".format("Dan", 40)
• Old style (%): "Name: %s, Age: %d" % ("Dan", 40)