Text Is Not a Single Thing

Index a string with square brackets, the same way you would a list, and counting starts at 0, not 1.

$ >>> name = "python"$ >>> name[0]$ 'p'$ >>> name[5]$ 'n'

name[0] is the first character precisely because Python counts positions, not places in line — position 0 is where you start counting from, and it happens to hold the first character.

Negative indices count backwards from the end, so you never need to know a string's length just to reach its last character.

$ >>> name[-1]$ 'n'$ >>> name[-6]$ 'p'

Two of the ordinary arithmetic operators work on strings too, because a string is a sequence and sequences can be joined and repeated. + concatenates; * repeats.

$ >>> "py" + "thon"$ 'python'$ >>> "ab" * 3$ 'ababab'

A slice, text[start:end], pulls out a whole range at once. The part everyone gets wrong at least once: end is not included.

$ >>> word = "python"$ >>> word[0:2]$ 'py'$ >>> word[2:6]$ 'thon'$ >>> word[:3]$ 'pyt'$ >>> word[-3:]$ 'hon'

word[0:2] stops before position 2, so it hands back two characters, not three. Leave out start and the slice begins at the front; leave out end and it runs to the finish. Because the end is excluded, word[:3] + word[3:] always rebuilds the whole string with nothing doubled and nothing lost.

Building a message out of a string and a variable used to mean concatenating pieces by hand, watching the types and the spacing yourself. An f-string does the whole job in place.

$ >>> name = "Ada"$ >>> age = 28$ >>> f"{name} is {age} years old"$ 'Ada is 28 years old'

The f before the opening quote is what turns the braces from literal characters into holes Python fills in. Anything that evaluates to a value can go inside them — not just a variable name, but a full expression.

$ >>> price = 19.5$ >>> f"Total: {price * 2:.2f}"$ 'Total: 39.00'

The part after the colon, .2f, is a format spec — “fixed-point, two decimal places”. It runs on the value after the expression is evaluated, which is why it can turn a plain 39.0 into the two-decimal 39.00 a price actually needs to display.

A string comes with dozens of built-in methods. Most of what you write day to day leans on a small handful of them.

$ >>> "  Ada Lovelace  ".strip()$ 'Ada Lovelace'$ >>> "Ada Lovelace".split()$ ['Ada', 'Lovelace']$ >>> "-".join(["Ada", "Lovelace"])$ 'Ada-Lovelace'

Notice the shape all five share: every one of them returns a new value instead of announcing what it did. Nothing prints unless you print it, and nothing changes unless you keep the result — which is exactly the next section.

Try to change one character of a string directly, and Python refuses outright.

$ >>> name = "python"$ >>> name[0] = "P"$ TypeError: 'str' object does not support item assignment

Strings are immutable — once created, a string never changes. Every method that looks like it edits one, such as .upper() or .replace(), actually builds and returns a brand new string, leaving the original exactly as it was.

$ >>> name = "python"$ >>> name.upper()$ 'PYTHON'$ >>> name$ 'python'

Immutability is not an arbitrary restriction. It is what lets Python use a string safely as a dictionary key, or hash it at all — a value that could silently change size or content out from under you would break every dictionary relying on it staying exactly as it was the moment it was stored.

Everything so far has treated a string as a sequence of characters. Underneath, a file on disk is not characters — it is bytes, and something has to agree on how those bytes map back to text. That agreement is an encoding, and getting it wrong is one of the most common ways a beginner's program crashes on someone else's machine but not their own.

$ >>> "café".encode("utf-8")$ b'caf\xc3\xa9'$ >>> "café".encode("utf-8").decode("ascii")$ UnicodeDecodeError: 'ascii' codec can't decode byte 0xc3 in position 3: ordinal not in range(128)

.encode() turns text into bytes under a given encoding; .decode() turns bytes back into text, and it has to be told the same encoding the bytes were written in, or it guesses wrong. é is one character but two bytes in UTF-8 — ascii, an older encoding that only covers the first 128 characters, has no representation for either of them.