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Strings are the most common data type you'll work with. User input, file names, web page content, database records, log messages — they're all strings. Python gives you a rich set of tools for working with text. This lesson covers the foundational operations: measuring, indexing, slicing, concatenating, repeating, and searching.

Strings are sequences of characters

A string is just an ordered sequence — like a list of single-character pieces, but immutable. Each position has an index:

  P  y  t  h  o  n
  0  1  2  3  4  5
 -6 -5 -4 -3 -2 -1

• Indices count from 0 forward.
• Negative indices count from the end backward (-1 is the last).
• Out-of-range indexing raises IndexError.

len() — how long is it

Slicing — extract a substring

Note: out-of-range slice indices DON'T error — Python clamps them. "abc"[0:100] returns "abc". This is opposite to indexing behavior!

Concatenation and repetition

+ joins strings. * with an integer repeats. You CAN'T do "a" + 1 — you'd need "a" + str(1) or use an f-string.

Membership and counting with in

For counting: use the method "banana".count("a") → 3.

Strings are IMMUTABLE

You cannot change a character in place:

Instead, build a NEW string:

Every "modification" to a string really creates a new string object. This is why concatenating thousands of strings in a loop with += is slow — it creates and discards many intermediate strings. For heavy concatenation, build a list and "".join(parts).

Iterating over a string

A for loop on a string yields one character at a time.

Multi-line strings

Triple quotes (single OR double, but matched) span multiple lines:

The newlines inside become real \n characters in the string.

Escape sequences

Inside a string, certain backslash sequences mean special characters:

• \n — newline
• \t — tab
• \\ — a literal backslash
• \" — a literal double quote inside "..."
• \' — a literal apostrophe inside '...'

Common mistakes

• Trying to mutate a string: s[0] = "X" raises TypeError. Build a new string instead.
• Off-by-one in slices: s[0:3] is THREE chars (0, 1, 2), not four. The stop is exclusive.
• Forgetting that * needs an integer, not a string: "a" * "3" is a TypeError.
• Confusing len() with the highest index: len("abc") is 3 but the highest valid index is 2.