Strings

Text values and literal notation

A string is Python's built-in representation for textual data. A program usually creates one with a quoted literal, such as 'hello' or "hello". Choosing one quote style over the other can make punctuation easier to express: "don't" needs no backslash around its apostrophe, while 'She said "yes"' needs no backslash around the double quotes. The quote marks are source notation; they delimit the value rather than becoming part of it. A string can be empty ('') and can contain one character. Python does not introduce a separate character type for that one-character case: 'P' is a string whose length is one.

Backslash begins an escape sequence inside an ordinary string literal. For example, '\n' represents a newline, '\t' represents a tab, and '\' represents one backslash. This lets source code describe characters that would otherwise be awkward to place directly in a literal. Python also recognizes Unicode escape forms; for example, '\N{GREEK SMALL LETTER PI}' evaluates to π. This lesson concerns the text value after Python evaluates a literal, not the encoding of a source file or the mechanics of reading text from a file.

A string is an immutable sequence

Strings are sequences: their characters have an order and positions. The first character has index 0, so in word = 'Python', word[0] is 'P'. Negative indices count back from the end: word[-1] is 'n'. Asking for one position with an index returns a one-character string. An index outside the available positions raises IndexError, which is a useful signal that a calculation selected no character.

A slice uses a colon to select a run of positions. In word[1:4], the start position 1 is included and the stop position 4 is excluded, producing 'yth'. Omitting the start means the beginning; omitting the stop means the end. Thus word[:2] is 'Py' and word[4:] is 'on'. Thinking of a slice as boundaries between characters prevents a common off-by-one error: the right boundary is not included. Unlike a single out-of-range index, a slice can extend beyond the available boundaries and is clipped by the sequence rules.

Immutable means the characters of an existing string cannot be replaced in place. word[0] = 'J' raises TypeError. To get different text, calculate a new string, perhaps with a slice and concatenation. This does not make strings unusable; it makes their existing character sequence stable. Do not confuse immutability with a rule that a program can never use a different value later. A name may be bound to a new string, but that is distinct from altering characters within an already existing string.

Building text with operations and f-strings

The + operator concatenates strings: 'Ada' + ' Lovelace' produces 'Ada Lovelace'. The space belongs in the second literal; concatenation does not automatically insert punctuation or spaces. This is a direct way to assemble a small fixed message. It is also a place to inspect types carefully: + joins two strings here, while adding numbers is covered separately.

An f-string is a string literal with an f prefix, such as f'{name} has {items} tasks.'. Braces introduce replacement fields containing expressions. When Python evaluates the f-string, it evaluates each expression and incorporates its result into the resulting text. With name = 'Ada' and items = 3, that example produces 'Ada has 3 tasks.'. The prefix is what makes braces special in this form; ordinary string literals do not evaluate text between braces. Use f-strings when a message has fixed surrounding wording and a few values to insert.

A disciplined trace follows each operation. For 'Python'[1:4], mark positions 1, 2, and 3, then stop before 4: y, t, h. For f'{2 + 3} items', calculate the expression first, then place 5 into the sentence. These small traces establish a reliable habit before later lessons add methods, input, or more complex data structures.