Python Strings and Functions Reference: Methods & Examples

Posted on Feb 1, 2026 in Telecommunication Technologies

Python String Cheat Sheet

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Python String Cheat Sheet
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Strings are

  • Indexed
  • Iterable
  • Immutable

Indexing & Slicing

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s = "python"

s[0]      -> 'p'
s[-1]     -> 'n'

Slicing:
s[start:end]       -> s[1:4] = 'yth'
s[:3]               -> 'pyt'
s[3:]               -> 'hon'
s[::2]              -> 'pto'
s[::-1]             -> reversed string

Important String Methods

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Case

  • s.upper()
  • s.lower()
  • s.capitalize()
  • s.title()

Search

  • s.find("x") → index or -1
  • s.rfind("x") → search from right
  • s.index("x") → raises an error if not found

Replace

s.replace("old", "new")

Strip (remove whitespace or characters)

  • s.strip()
  • s.lstrip()
  • s.rstrip()
  • s.strip("x") → removes ‘x’ from both ends

Count

s.count("a")

Check types

  • s.isalpha()
  • s.isdigit()
  • s.isalnum()
  • s.isspace()

Split & Join

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Split

"a,b,c".split(",")     -> ['a','b','c']
"hi there".split()       -> ['hi','there']

Join

",".join(['a','b','c'])   -> 'a,b,c'
" ".join(['hi','there'])  -> 'hi there'

String Formatting

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f-strings (preferred)

name = "Dan"
f"Hello {name}"
f"{3.14159:.2f}"     -> '3.14'

format()

"{} {}".format("A", "B")
"{1} {0}".format("A", "B")   -> 'B A'

Old-style formatting

"%d" % 42
"%s" % "hello"
"%.2f" % 3.14

String Immutability

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# NO:
s[0] = "X"   -> ERROR

# YES: create a new string
s = "X" + s[1:]

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Python Functions
===========================

Python Functions Cheat Sheet

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Function Basics

def name(params):
    statements
    return value

Functions must be defined before use.

Call

f(3, 4)

Return vs Print

  • print() shows output
  • return sends a value back
  • A function with no return returns None

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Default Parameters

def f(a, b=5):
    return a + b

f(3) → 8
f(3, 2) → 5

Rules:

  • Positional arguments first
  • Keyword arguments after
  • Default parameters must come after non-defaults

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*args (variable number of positional arguments)

def f(*args):
    print(args)

f(1,2,3) → (1, 2, 3) # tuple

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**kwargs (variable keyword arguments)

def f(**kwargs):
    print(kwargs)

f(a=1, b=2) → {'a': 1, 'b': 2} # dict

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Multiple Return Values

def stats(x, y):
    return x+y, x*y

a, b = stats(3,4)
# returns a tuple

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Scope (LEGB rule)

Local (inside function)
Enclosing
Global (defined top-level)
Built-in

Local variables shadow global variables.

global keyword

x = 10
def f():
    global x
    x = 5

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Functions Are Objects

Functions can be:

  • assigned to variables
  • stored in lists/dictionaries
  • passed as arguments
x = f
x()

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Common Errors

  • Missing colon after def
  • Indentation errors
  • Calling before definition
  • Using undefined variables
  • Returning print() (returns None)

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Docstrings

def f(x):
    """This function does something."""
    return x

help(f) shows the docstring.

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Function Stubs

def f():
    pass

Placeholder function that does nothing.

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Namespace & Scope Resolution

LEGB:
Local > Enclosing > Global > Built-in

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Arbitrary Argument Lists

*args → tuple
**kwargs → dictionary

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Functions with Loops / Branches

def f(n):
    for i in range(n):
        print(i)

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Dynamic Typing in Functions

Parameters accept any type:
def add(a, b):
    return a + b    # works for ints, strings, lists, etc.

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Print Functions

Functions can print instead of returning:
def show(x):
    print(x)

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Reasons for Defining Functions

  • Avoid repeating code (DRY)
  • Reusable
  • Organized
  • Easier testing and debugging