Strings

Python - A Quick Start for existing Programers

1 min read

Published Sep 16 2025, updated Sep 30 2025

Python

Creating strings

s1 = 'single quotes'

s2 = "double quotes"

s3 = '''triple quotes for multi-line'''

Basic string operations

s = "Hello World"

print(len(s))

# length

print(s[0])

# indexing (H)

print(s[-1])

# last character (d)

print(s[0:5])

# slicing ("Hello")

print("Hello" in s)

# membership (True)

print(s + "!")

# concatenation

print(s * 3)

# repetition

Common string methods

txt = " Python is Fun! "

print(txt.lower())

# " python is fun! "

print(txt.upper())

# " PYTHON IS FUN! "

print(txt.title())

# " Python Is Fun! "

print(txt.strip())

# "Python is Fun!" (remove spaces)

print(txt.replace("Fun", "Awesome"))

# " Python is Awesome! "

print(txt.split())

# ['Python', 'is', 'Fun!']

print(",".join(["a","b","c"]))

# "a,b,c"

print(txt.find("Fun"))

# index of substring

print(txt.startswith("Py"))

# False

print(txt.endswith("!"))

# True

f-strings

Adding an f before a string allows you to add variables in the string which are resolved and can be formatted

Width & alignment: <, >, ^, = (padding before sign)
Fill character: {value:*^10}
Signs: +, -, (space for positives)
Numbers: .nf (decimals), , (comma), _ (underscore), b (binary), o (octal), x/X (hex), e/E (scientific), % (percentage)
Datetime: use strftime codes inside {}
Debugging: {var=}

Examples:

name = "Alice"

age = 30

print(f"My name is {name} and I am {age} years old")

print(f"In 5 years, I’ll be {age+5}")

num = 1234.56789

print(f"Default: {num}")

print(f"Two decimals: {num:.2f}")

# 1234.57

print(f"Comma separator: {num:,}")

# 1,234.56789

print(f"Percent: {0.25:.0%}")

# 25%

print(f"Scientific: {num:.2e}")

# 1.23e+03

text = "Hi"

print(f"[{text:<10}]")

# Left-align within 10 spaces -> [Hi ]

print(f"[{text:>10}]")

# Right-align within 10 spaces -> [ Hi]

print(f"[{text:^10}]")

# Center -> [ Hi ]

print(f"[{text:*^10}]")

# Center, fill with * -> [****Hi****]

num = 42

print(f"[{num:5}]")

# Width 5 -> [ 42]

print(f"[{num:05}]")

# Zero-padding -> [00042]

print(f"[{num:+}]")

# Always show sign -> [+42]

print(f"[{-num:+}]")

# [-42]

print(f"[{num: }]", f"[{-num: }]")

# Space for positive -> [ 42] [-42]

pi = 3.1415926535

print(f"{pi:.2f}")

# 3.14 (2 decimals)

print(f"{pi:.4f}")

# 3.1416

print(f"{pi:10.2f}")

# Field width 10, 2 decimals -> " 3.14"

num = 1234567.89

print(f"{num:,}")

# 1,234,567.89

print(f"{num:_}")

# 1_234_567.89 (Pythonic underscore style)

num = 255

print(f"{num:b}")

# binary -> 11111111

print(f"{num:o}")

# octal -> 377

print(f"{num:x}")

# hex lowercase -> ff

print(f"{num:X}")

# hex uppercase -> FF

rate = 0.256

print(f"{rate:.0%}")

# 26%

print(f"{rate:.2%}")

# 25.60%

num = 12345.6789

print(f"{num:.2e}")

# 1.23e+04

print(f"{num:.3E}")

# 1.235E+04

from datetime import datetime

now = datetime(2025, 9, 15, 14, 30)

print(f"{now:%Y-%m-%d %H:%M}")

# 2025-09-15 14:30

x = 10

print(f"{x=}")

# x=10

Escape Sequences

print("Line1\nLine2")

# newline

print("Tab\tSpace")

# tab

print("Quote: \"Hello\"")

# escape quotes

print("Backslash: \\")

# backslash

String checking methods

s = "Python3"

print(s.isalpha())

# False (contains digit)

print(s.isdigit())

# False

print(s.isalnum())

# True (letters + digits)

print(s.isspace())

# False

print("HELLO".isupper())

# True

print("hello".islower())

# True

String comprehensions

Python can use list comprehensions on strings.
Use "".join(...) to rebuild a string after transformation/filtering.
You can do:
- Transformations (uppercase, doubling)
- Filtering (keep only certain chars)
- Conditional replacements (mask vowels/digits)

Examples:

text = "Python 3.12!"

# ----------------------------------------

# Basic transformations

# ----------------------------------------

# Uppercase all characters

upper_str = "".join([ch.upper() for ch in text])

print("Uppercase:", upper_str)

# Double each character

doubled = "".join([ch*2 for ch in text])

print("Doubled chars:", doubled)

# ----------------------------------------