Strings & f-Strings

Strings Are Immutable Sequences

Strings support indexing and slicing like lists, but every “modification” returns a new string:

s = "Coding India"
s[0]          # 'C'
s[-5:]        # 'India'
s.upper()     # 'CODING INDIA' — s itself is unchanged

f-Strings — the Only Formatting You Need

name, subs = "Coding India", 10500

f"{name} has {subs} subscribers"
f"{subs:,}"            # '10,500'  — thousands separator
f"{3.14159:.2f}"       # '3.14'    — 2 decimal places
f"{0.876:.1%}"         # '87.6%'   — percentage
f"{42:>8}"             # '      42' — right-align in 8 chars
f"{name=}"             # "name='Coding India'" — debug shorthand

Any expression works inside the braces: f"total: {price * qty}".

The Methods You’ll Use Every Day

"  hello  ".strip()              # 'hello'
"Hello".lower() / .upper()
"hello world".title()            # 'Hello World'
"hello".replace("l", "L")        # 'heLLo'
"hello".startswith("he")         # True
"data.csv".endswith(".csv")      # True
"hello".find("ll")               # 2 (or -1 if missing)
"hello".count("l")               # 2
"42".isdigit()                   # True

Split & Join — Text ⇄ Lists

"django,react,fastapi".split(",")
# ['django', 'react', 'fastapi']

"  multiple   spaces ".split()        # splits on any whitespace
# ['multiple', 'spaces']

", ".join(["django", "react"])        # 'django, react'
"-".join(str(n) for n in [2, 0, 2, 6])  # '2-0-2-6'

Note the direction: join is a string method that takes a list — the separator comes first.

Multiline Strings & Raw Strings

query = """
SELECT name, score
FROM students
WHERE score > 80
"""

path = r"C:\Users\new_folder"    # r'' = raw — backslashes left alone
pattern = r"\d{4}-\d{2}-\d{2}"   # essential for regex

Building Strings in Loops

Because strings are immutable, += in a loop copies the whole string every iteration. Collect parts in a list and join once:

parts = []
for row in rows:
    parts.append(render(row))
html = "".join(parts)            # one allocation, fast