Python · Beginner

Strings in Python

In short: A Python string (str) is an immutable sequence of characters written in single or double quotes. You read characters by index, take parts with slices, join with +, and transform text with methods such as upper(), strip(), split() and replace(), each of which returns a new string because the original can never change.

Text as a sequence

Text is the most common kind of data a program handles: names, addresses, lines of a file, everything input() returns. Python stores text in the str type. A string literal is written between matching quotes, single or double; the choice makes no difference to the value, which lets you write "it's" without escaping anything. Triple quotes allow a literal to span several lines. Inside a string, \n stands for a line break and \t for a tab.

A string behaves as a sequence. len(s) counts its characters, s[0] is the first character and s[-1] the last. A slice s[start:stop] takes characters from start up to but not including stop; leaving out start means from the beginning, leaving out stop means to the end, and s[::-1] reverses. Slicing never fails on out-of-range bounds, it just clips, whereas indexing a single position past the end raises IndexError. A for loop over a string visits one character per round, and in tests whether one string appears inside another.

Strings are immutable. s[0] = "X" is a TypeError. Every operation that seems to modify a string, from upper() to replace(), actually builds and returns a new one, so the result must be assigned somewhere: name = name.strip(). Immutability is what makes strings safe to share between parts of a program and to use as dictionary keys.

The str type has dozens of methods; a handful cover most work. strip() removes surrounding whitespace, which matters for input. lower() and upper() change case, usually so that a comparison ignores it. split() cuts a string into a list of words, or on a separator you pass, and ", ".join(items) does the reverse. replace(old, new) swaps every occurrence. startswith(), endswith(), find() and count() ask questions about content.

For building text, f-strings (Python 3.6 and later) are the tool: an f before the opening quote lets you put expressions in braces, as in f"{qty} x {name}", with an optional format spec after a colon. {price:.2f} prints two decimals and {n:>5} right-aligns in five columns. They replace error-prone chains of + and str().

Syntax

s = "text"   or   'text'        # the same value
s[i]         s[-1]              # index from the front / from the end
s[a:b]       s[a:]   s[:b]      # slice: a up to (not including) b
len(s)       "sub" in s
s.strip()    s.lower()   s.upper()
s.split()    s.split(",")   ", ".join(parts)
s.replace("old", "new")   s.startswith("x")   s.count("e")
f"{value} and {price:.2f}"       # f-string with a format spec

Every method returns a new string; the original is unchanged. join() is called on the separator and given the list of parts.

Common string methods

Indexing and slicing a booking code

One twelve-character code, taken apart by position.

code = "LHR-DEL-0725"
print(len(code))
print(code[0], code[-1])
print(code[0:3])
print(code[4:7])
print(code[-4:])
print(code[::-1])
print("DEL" in code, "SFO" in code)
for ch in code[:3]:
    print(ch, end=".")
print()

12
L 5
LHR
DEL
0725
5270-LED-RHL
True False
L.H.R.

Positions count from 0, so code[0:3] is the first three characters and code[4:7] skips the hyphen at index 3. Negative indexes count from the end, so code[-4:] is the last four characters whatever the length. The step of -1 in code[::-1] walks backwards and reverses the string. The loop shows that a slice is an ordinary string you can iterate over.

Cleaning and inspecting input

The input line is Green Tea, large with two spaces on each side.

order = input()
print("[" + order + "]")
order = order.strip()
print("[" + order + "]")
print(order.upper())
print(order.lower().replace(" ", "_"))
parts = order.split(", ")
print(parts)
print(order.startswith("Green"), order.count("e"), order.find("Tea"))

[  Green Tea, large  ]
[Green Tea, large]
GREEN TEA, LARGE
green_tea,_large
['Green Tea', 'large']
True 4 6

The brackets make the stray spaces visible; strip() removes them and the result is assigned back, because the original string is not changed. Methods can be chained, as lower().replace() shows, since each returns a new string to call the next method on. split(", ") cuts on the exact separator and gives a list, which the lists lesson picks up. Note that order is unchanged by upper(): it still starts with a capital G on the last line, where find reports that "Tea" begins at index 6.

Building text with f-strings and join

Formatting a receipt line, a two-column header and a comma-separated list.

item = "notebook"
price = 3.5
quantity = 4
print(f"{quantity} x {item} at {price:.2f} each")
print(f"Total: {quantity * price:.2f}")
print(f"{'Item':<10}|{'Qty':>4}")
print(f"{item:<10}|{quantity:>4}")
colours = ["red", "amber", "green"]
print(", ".join(colours))
print("-" * 12)
print("ab" + "cd", "ha" * 3)

4 x notebook at 3.50 each
Total: 14.00
Item      | Qty
notebook  |   4
red, amber, green
------------
abcd hahaha

Anything inside the braces is evaluated, including quantity * price. The format spec after the colon controls the appearance: .2f fixes two decimals, <10 pads to ten characters left-aligned, >4 right-aligns in four. join is called on the separator and given the list. + concatenates and * repeats, which is a quick way to draw a rule line.

Common mistakes

• Calling a method and ignoring the result

  Why it goes wrong: name.strip() on its own line computes a new string and throws it away; name still has its spaces, because strings cannot change in place.

  Fix: Assign the result: name = name.strip().

   Python · fixCopy

  name = "  Ola "
  name = name.strip()
  print(len(name))  # 3

• Trying to change one character by index

  Why it goes wrong: s[0] = "J" raises TypeError: 'str' object does not support item assignment.

  Fix: Build a new string from slices: s = "J" + s[1:].

• Joining text and numbers with +

  Why it goes wrong: "Age: " + 30 is a TypeError; + on a string only accepts another string.

  Fix: Use an f-string, f"Age: {age}", or pass separate arguments to print().

• Forgetting that the slice end is excluded

  Why it goes wrong: s[0:3] gives three characters, at indexes 0, 1 and 2, not four. Reading it as "from 0 to 3 inclusive" is a frequent source of missing last characters.

  Fix: Remember that s[a:b] has length b - a, and that s[:3] + s[3:] is always the whole string.

Where you use this

Comparing user input reliably needs normalising first: answer.strip().lower() in ("y", "yes") accepts "Yes", " YES " and "y" alike. Parsing a line such as Nairobi,14.5,cloudy is split(",") followed by converting the middle piece to float. Producing a report is the reverse: f-strings with width and precision specs line up columns so the output is readable, and join builds a comma-separated line from a list of values. Every exercise that reads text and prints text passes through these few methods.

line = "Nairobi,14.5,cloudy"
city, temp, sky = line.split(",")
print(f"{city:<10}{float(temp):>6.1f}  {sky}")

Key points

• Strings are sequences: index with [i], slice with [a:b], loop with for, test with in.
• Strings are immutable; every method returns a new string, so assign the result.
• strip(), lower(), split() and replace() cover most input clean-up.
• ", ".join(list) turns a list into one string; split() goes the other way.
• f-strings embed expressions and format specs: f"{price:.2f}".
• Slices clip at the ends; single-index access past the end raises IndexError.

Try it yourself

Read a full name and print its initials in upper case, each followed by a full stop, so that nora kaspar vale becomes N.K.V.. Use split() and indexing.

name = input()
# build the initials and print them

name = input()
initials = ""
for part in name.split():
    initials += part[0].upper() + "."
print(initials)

Practise this

• Dispatch message initialsEasy

  Create uppercase initials from a multi-word dispatch message using Python string operations.

  StringsNot started

Open the Python playground

Related lessons

• Data Types in PythonThe core Python data types int, float, str, bool and None: how to check a type with type() and convert between types with int(), float() and str().8 minCompleted
• Lists in PythonHow Python lists store ordered, changeable collections: creating them, indexing and slicing, adding and removing items, sorting, copying and looping over them.9 minCompleted
• Loops in Python: for and whileHow a for loop walks through a sequence and a while loop repeats until a condition fails, with range(), enumerate(), break and continue explained by example.9 minCompleted

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