CODESARRAY

Lesson 4, by Codesarray, , 5 min read

Python strings: indexing, slicing, methods and f-strings

Pick characters out of a Python string, slice pieces off it, clean it up with methods like strip and split, and build new text with f-strings.

Python strings are text you can index, slice, clean up and format. By the end of this page you will pick characters out of a string, slice pieces off it, tidy it with methods like strip() and split(), and build new text with f-strings.

Index and slice a string

A string is a sequence of characters, and each one has a position counted from zero. Negative positions count from the end, and len() tells you how many characters there are. A slice takes a piece: word[0:3] runs from 0 up to, but not including, 3. Leave out the start and it begins at the beginning, leave out the end and it runs to the end. A third number is the step, and a step of -1 walks backwards:

word = "Python"
print(word[0], word[-1])
print(word[0:3], word[2:])
print(word[::-1])
P n
Pyt thon
nohtyP

word[0] is the first letter and word[-1] the last. word[0:3] is Pyt, word[2:] is thon, and word[::-1] is the whole word reversed.

Strings are immutable

You cannot change a character in place:

word[0] = "J"
TypeError: 'str' object does not support item assignment

A string never changes after it is made. Every method that looks like it changes one actually returns a new string. word.upper() gives 'PYTHON', but word is still 'Python'. If you want the new value, assign it back, as the next section does with name = name.strip().

Python string methods: strip, split, replace

Strings come with dozens of methods. strip() removes spaces at both ends, upper() and lower() change the case, split() cuts a string into a list at a separator, join() glues a list back together with the string you call it on, and replace() swaps one piece of text for another. The in test tells you whether a piece is there at all:

name = "  Ada Lovelace  "
name = name.strip()
print(name.upper())
print(name.split(" "))
print(name.replace("Ada", "A."))
ADA LOVELACE
['Ada', 'Lovelace']
A. Lovelace

Because strings are immutable, line 2 assigns the stripped text back to name. The three prints then work on the cleaned string and leave it unchanged.

Build text with f-strings

To build text from values, put an f before the quotes. Anything in curly braces is evaluated and dropped into the text, and any expression works, such as {age + 1}. After a colon you say how to format it: :.2f rounds to two decimal places and :, adds thousands separators. Put an equals sign inside the braces and Python prints the name and the value together, which is handy for debugging:

age = 36
print(f"{name} is {age}")
print(f"{age=}")
print(f"{age / 7:.2f}")
Ada Lovelace is 36
age=36
5.14

name still holds Ada Lovelace from the last section. f"{age=}" printed age=36, and age / 7 is 5.142857..., rounded to 5.14.

A glance at t-strings

Python 3.14 added t-strings: the same syntax with a t instead of an f. A t-string does not give you text. It gives a Template object that keeps the pieces of text and the values separate, so a library can escape them safely before building HTML or SQL:

name = "Ada"
t = t"Hello {name}"
print(t.strings, t.values)
('Hello ', '') ('Ada',)

You do not need them yet. For everyday text, f-strings are what you want.

What you learned

  • word[0] picks a character and negative numbers count from the end.
  • word[1:4] slices a piece, and a step of -1 reverses the string.
  • Strings are immutable: methods return new strings, so assign the result back.
  • strip(), split(), join() and replace() do most of the everyday cleaning.
  • f-strings put values into text, with a format after a colon and {x=} for debugging.

Common questions

What happens if I index past the end? "Python"[10] raises IndexError: string index out of range. A slice is more forgiving: "Python"[2:100] gives 'thon' and "Python"[10:] gives an empty string.

What is the difference between split() and split(" ")? With no argument, split() treats any run of spaces as one gap, so "a b c".split() gives ['a', 'b', 'c']. split(" ") cuts at every single space and keeps the empty pieces: ['a', '', 'b', '', '', 'c'].

Why does "age: " + 36 fail? Python will not guess how to join text and a number. It raises TypeError: can only concatenate str (not "int") to str. Use an f-string instead: f"age: {36}".

Does len() count letters or bytes? It counts characters, so len("héllo") is 5 and len("🐍") is 1.

How do I remove spaces from only one end? Use lstrip() for the left end and rstrip() for the right. Give any of the three a string such as "xxhixx".strip("x") and it removes those characters instead of spaces.

This lesson follows on from Python if else and loops. Next in the series: lists.

This article goes with the video Strings. Watch it on YouTube.