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Strings

Introduction

A string is a piece of text data. Strings are used frequently in programming, for example to display messages to the user, or to represent filenames or web URLs, and many other things.

About this lesson

In this lesson we will look at: * Declaring strings * The input function * Converting between strings and other data types * How to use special characters in a string

Declaring strings

You can declare a string by enclosing the text in quotation marks. You can use either single quote characters ‘ or double quote characters “. It really doesn’t matter which you use, but whichever character you use to start the string you must use the same character to end it:

a = 'Hello'
b = "world"
c = 'Not valid"  # Must start and end with same type

Python traditionally uses single quotes, but it really doesn’t matter if you prefer to use double quotes.

The input function

The input function provides a simple way to request user input from the console:

name = input('What is your name?')
print('Hello', name)

The input statement displays a prompt and waits for the user to type something in:

>>> What is your name?

If you type in your name and hit enter, the second line of the program (the print statement) will display a greeting:

>>> What is your name? Martin
Hello Martin

Converting between strings and other data types

You can convert almost any type of data to a string, using the str function. Try this using the Python console:

>>> str(1 + 2)
'3'
>>> str(1 / 2)
'0.5'

Here we calculate an integer value and convert it to a string. Thenm we calculate a float value and convert it to a string. In fact, str works on almost any type of value.

You can convert numbers to strings too, using the int and float functions:

>>> int('5')
5
>>> float('2.3')
2.3
>>> float('abc')
ValueError

If you pass a string in that cannot be converted to a number, you will get an error - a ValueError.

Operations on strings

Python allows you to perform all sorts of functions on strings. We will meet more of these later (they will make more sense once you know an#bout lists and slices), but here are a few simple operations.

You can join two strings into one:

>>> 'Hello' + 'World'
'HelloWorld'

You can also repeat a string n times:

>>> 'Hello' * 3
'HelloHelloHello'

There are lots of string functions, here are a selection:

>>> s = 'Hello, world!'
>>> s.upper()
'HELLO, WORLD!'           (converts to upper case)
>>> s.center(20)
'   Hello, world!    '    (pads to 20 characters, with text in centre)
>>> s.split()
['Hello,', 'world!']      (splits into separate words)

The last example produces a list of the separate words in the string - we will meet lists soon!

We will meet more string functions in this later in this course.

How to use special characters in a string

Sometimes you might need to include “special” characters in a string.

Quote characters

For example, what if your string contained a quote character?

s = 'This isn't easy'    # ERROR

Unfortunately, this gives an error. The problem is that the ‘ character in isn’t tells Python that the string has ended. So what Python sees is:

  • the string ‘This isn’
  • random characters t easy’ after the string, which aren’t valid Python.

There are two ways to fix this:

  • You can use the \ character, called the escape character, to tell Python that the following character needs special treatment. ie treat it as part of the string, not the end of the string.
  • You can use enclose the string in double quotes. In that case, a single quote character isn’t anything special so gets handled normally.
u = 'This isn\'t easy'
v = "This isn't easy"

Both methods are equally valid. You can use similar methods if you need to include double quotes in a string:

w = "Using \"special\" characters"
x = 'Using "special" characters'

The escape character

The character \ has a special meaning - it is the escape character. But what if you need to use that actual character in your string? Well, you have to escape it. You use \\ to represent \.

s = 'How to use \\ in a string'     # How to use \ in a string

Unicode characters

Python supports Unicode, which means it can handle just about every character and symbol you can imagine. That includes every character used by almost any language in the world, plus a whole variety of mathematical and scientific symbols, and lots more besides.

Normally you can just paste special characters into your Python editor direct. For example if we want to print the temperature:

t = 18
print('The temperature is', t, '° C')

If you just copy and paste that code into IDLE, the degree symbol will be copied in and will work just fine.

An alternative way, particularly if your editor doesn’t support Unicode, is to use the character code. For example the Unicode square root symbol is √. A quick web search for “unicode square root” will locate it, and tell you that its Unicode character code is 221A (hex).

print('The square root symbol is \u221a')

The escape code \u followed by the 4 digit unicode value adds the character to the string. Make sure you have the unicode value not the decimal value (for example square root has hex value 221a, which is equal to decimal value 8,730 - you must use the hex value).