Strings

Python - A Quick Start for existing Programers

1 min read

Published Sep 16 2025, updated Sep 30 2025

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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)

# ----------------------------------------
# Filtering characters
# ----------------------------------------
# Keep only letters
letters = "".join([ch for ch in text if ch.isalpha()])
print("Letters only:", letters)

# Keep only digits
digits = "".join([ch for ch in text if ch.isdigit()])
print("Digits only:", digits)

# Remove spaces
no_spaces = "".join([ch for ch in text if ch != " "])
print("No spaces:", no_spaces)

# ----------------------------------------
# Conditional replacements
# ----------------------------------------
# Replace vowels with '*'
mask_vowels = "".join(["*" if ch.lower() in "aeiou" else ch for ch in text])
print("Mask vowels:", mask_vowels)

# Replace digits with '#'
mask_digits = "".join(["#" if ch.isdigit() else ch for ch in text])
print("Mask digits:", mask_digits)

# ----------------------------------------
# Combining rules
# ----------------------------------------
# Only consonants, all uppercase
consonants = "".join([ch.upper() for ch in text if ch.isalpha() and ch.lower() not in "aeiou"])
print("Consonants uppercase:", consonants)

# Reverse string via comprehension
reversed_str = "".join([text[i] for i in range(len(text)-1, -1, -1)])
print("Reversed:", reversed_str)




Raw Strings (ignore escapes)

path = r"C:\Users\Alice\Docs"
print(path)
# C:\Users\Alice\Docs
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