Strings: looking inside

A string is a sequence of characters, and Python lets you look at any part of it. This page is about reading a string: its length, its characters, and pieces of it.

Length

len gives the number of characters, spaces included.

item = "sourdough bread"
print(len(item))
print(len(""))

Indexing

Each character has a position, starting from 0. Square brackets pick one out.

item = "bread"
print(item[0])
print(item[1])
print(item[4])

Positions run from 0 to len(item) - 1. Asking for item[5] here is an IndexError. Negative positions count from the end: -1 is the last character, -2 the one before it.

item = "bread"
print(item[-1])
print(item[-2])

Slicing

item[start:stop] gives the characters from start up to but not including stop.

item = "sourdough"
print(item[0:4])
print(item[4:9])

Leaving out start means "from the beginning", leaving out stop means "to the end". So item[:4] is the first four characters and item[4:] is everything after them. Slices never raise an error for going past the end; they just stop.

item = "sourdough"
print(item[:4])
print(item[4:])
print(item[-3:])

Negative numbers work for stop too. item[:-2] means "everything except the last two characters", because -2 is the position of the second-to-last character and the slice stops before it.

item = "sourdough"
print(item[:-2])
print(item[1:-1])

A third number is the step: how far to jump each time. item[::2] takes every second character. A step of -1 walks backwards, so item[::-1] is the whole string reversed.

item = "sourdough"
print(item[::2])
print(item[::-1])

Is it in there?

in asks whether one string appears inside another. It gives a boolean, so it fits straight into an if.

item = "sourdough bread"
print("bread" in item)
print("milk" in item)
print(" " in item)

Joining and repeating

+ joins strings. * repeats one.

print("sour" + "dough")
print("-" * 20)

+ only joins strings with strings. To put a number into text, convert it with str first. The opposite, int, turns text like "42" into a number.

total = 120
print("Total: " + str(total))
print(int("42") + 1)

The next page shows a nicer way to build text from values.

Try it

First letter

Write first_letter(name) that returns the first character of name, in capitals. You can capitalise with .upper().

first_letter("bread")  ->  "B"
first_letter("Milk")   ->  "M"

name always has at least one character.

def first_letter(name):
    ...

Last three characters

Product codes end in a three-character batch number. Write batch(code) that returns the last three characters of code.

batch("SKU-1042-A7B")  ->  "A7B"
batch("BRD-001")       ->  "001"

code always has at least three characters.

def batch(code):
    ...

Has a space?

Write has_space(text) that returns True if text contains at least one space character, and False otherwise.

has_space("sourdough bread")  ->  True
has_space("bread")            ->  False

def has_space(text):
    ...

Next: String methods.