Lecture 5 — Python Strings


This material is drawn from Chapters 3 of Practical Programming and Chapter 8 of Think Python.

More Than Just Numbers

  • Much of what we do today with computers revolves around text:

    • Web pages
    • Facebook
    • Text message

    These require working with strings.

  • Strings are our third type, after integers and floats.

  • We’ve already started to use strings in our output, for example,

    def area_and_volume(radius, height):
        print "For a cylinder with radius", radius, "and height", height
        print "The surface area is ", area_cylinder(radius,height)
        print "The volume is ", volume_cylinder(radius, height)

Topics for Today

  • String basics
  • String operations
  • Input to and (formatted) output from Python programs

Strings — Definition

  • A string is a sequence of 0 or more characters delimited by single quotes or double quotes.

    "Albany, NY"
    '4 8 15 16 23 42'
  • We can print strings:

    >>> print "Hello, world!"
    Hello, world!
  • Strings may be assigned to variables:

    >>> s = 'Hello'
    >>> t = "Good-bye"
    >>> print s
    >>> t
  • Notice that unlike integers and floats there is now a difference between asking Python for the value of the variable and printing the variable!

Combining Single and Double Quotes in a String

  • A string that starts with double quotes must end with double quotes, and therefore we can have single quotes inside.

  • A string that starts with single quotes must end with single quotes and therefore we can have double quotes inside.

  • To illustrate this, we will take a look at

    >>> s = 'He said, "Hello, World!"'
    >>> t = "Many single quotes here '''''''  and here ''' but correct."

Multi-Line Strings

  • Ordinarily, strings do not extend across multiple lines, causing an error if you try.

  • But, starting and ending a string """ or ''' tells Python to allow the string to cross multiple lines.

    • Any character other than ''' (or """, if that is how the string started) is allowed inside the string.
  • Example,

    >>> s1 = """This
    is a multi-line
    >>> s1
    'This\nis a multi-line\nstring.'
    >>> print s1
    is a multi-line
  • Notice the \n when we ask Python for the value of the string (instead of printing it). This is an escape character, as we will discuss next.

Escape Characters

  • Inserting a \ in the middle of a string tells Python that the next character will have special meaning (if it is possible for it to have special meaning).

  • Most importantly:

    • \n — end the current line of text and start a new one
    • \t — skip to the next “tab stop” in the text. This allows output in columns
    • \' — do not interpret the ' as a string delimiter
    • \" — do not interpret the " as a string delimiter
    • \\ — put a true back-slash character into the string
  • We’ll explore the following strings in class

    >>> s0 = "*\t*\n**\t**\n***\t***\n"
    >>> s1 = "I said, \"This is a valid string.\""

Exercise Set 1

  1. Which of the following are valid strings? Fix the mistakes to make them all valid.

    >>> s0 = "Sheldon Cooper's apartment is in Pasedena"
    >>> s1 = 'This cheese shop's cheese is all gone"
    >>> s2 = """We are
    "The Knights of the Round Table"
    >>> s3 = "Toto, I said,\n"We aren't in Kansas, anymore!"
    >>> s4 = 'Have you seen the "Incredibly Photogenic Guy"'s picture?'
    >>> s5 = "Have you seen the 'Incredibly Photogenic Guy''s picture?"
  2. What is the output?

    >>> s = "Cats\tare\n\tgood\tsources\n\t\tof\tinternet\tmemes"
    >>> print s

String Operations — Concatenation

  • Concatenation: Two (or more) strings may be concatenated to form a new string, either with or without the + operator. We’ll look at

    >>> s0 = "Hello"
    >>> s1 = "World"
    >>> s0 + s1
    >>> s0 + ' ' + s1
    >>> 'Good' 'Morning' 'America!'
    >>> 'Good ' 'Morning ' 'America!'
  • Notice that

    >>> s0 = "Hello"
    >>> s1 = " World"
    >>> s0 s1

    is a syntax error but

    >>> "Hello" " World"

    is not. Can you think why?

String Operations — Replication

  • You can replicate strings by multiplying them by an integer:

    >>> s = 'Ha'
    >>> print s * 10
  • What do you think multiplying a string by a negative integer or 0 does? Try it.

  • Many expressions you might try to write involving strings and either ints or floats are illegal Python, including the following:

    >>> 8 * 'Hello'
    >>> 'Hello' * 8.1
    >>> '123' + 4

    Think about why

String Operations — Functions

  • You can compute the length of a string with len().
  • You can convert an integer or float to a string with str().
  • You can convert a string that is in the form of an integer to an integer using int()
  • You can convert a string that is in the form of a float to a float using, not surprisingly, float()
  • We will look at examples of all of these during lecture.

Exercise Set 2: String Operations

  1. What is the output of the following:

    >>> len('George')
    >>> len(' Tom  ')
    >>> s = """Hi
    >>> len(s)
  2. Which of the following are legal? For those that are, show what Python outputs.

    >>> 'abc' + str(5)
    >>> 'abc' * str(5)
    >>> 'abc' + 5
    >>> 'abc' * 5
    >>> 'abc' + 5.0
    >>> 'abc' + float(5.0)
    >>> str(3.0) * 3
  3. Write a line of code that prints 50 '*' characters.

  4. Write a function that takes a string as an argument and prints the string underlined with = equal to the length of the string. For example, we should have the following output:

    underline('Super Bowl')

    should output

    Super Bowl

    Use the len function and string replication.

String Output

  • We already know a bit about how to use print, but here are a few things to remember.
    • A space is added between each value that is output in a print statement
    • Each print statement starts a new line of output... unless the previous print statement ended with a ,
  • But, let’s look at some nicer ways to create output...

Formatted Output

  • In the Lecture 3 Python program area_volume.py, the last few lines are

    def area_and_volume(radius, height):
        print "For a cylinder with radius", radius, "and height", height
        print "The surface area is ", area_cylinder(radius,height)
        print "The volume is ", volume_cylinder(radius, height)
  • This produces the output

    For a cylinder with radius 5 and height 10
    The surface area is  471.2385
    The volume is  785.3975
  • Here is better formatting, without the insignificant values

    def area_and_volume(radius, height):
        print "For a cylinder with radius %d height %d" %(radius,height)
        print "The surface area is %.2f" %area_cylinder(radius,height)
        print "The volume is %.2f" %volume_cylinder(radius, height)

    which produces

    For a cylinder with radius 5 height 10
    The surface area is 471.24
    The volume is 785.40
  • We will discuss the significance of

    • %d
    • %.2f
    • %(radius,height)

User Input

  • Python programs can ask the user for input using the function call raw_input.

  • This waits for the user to type a line of input, which Python reads as a string.

  • This string can be converted to an integer or a float (as long as it is properly an int/float).

  • Here is a toy example

    print "Enter a number"
    x = float(raw_input())
    print "The square of %.1f is %.1f" %(x,x*x)
  • We can also insert the string right into the raw_input function call:

    x = float(raw_input("Enter a new number"))
    print "The square of %.1f is %.1f" %(x,x*x)
  • We will use this idea to modify our area and volume calculation so that the user of the program types in the numbers.

    • The result is more useful and feels more like a real program (albeit one run from the command line).
    • It will be posted on the course website.


  • Strings represent character sequences — our third Python type

  • String operations include addition (concatenate) and replication

  • We can concatenate by ’+’ or by using formatted strings:

    >>> 'a' + 'b'
    >>> '%d eggs and %s spam' %(2,'no')
  • Functions on strings may be used to determine length and to convert back and forth to integers and floats.

  • Escape sequences change the meaning of special Python characters or make certain characters have special meaning.

  • Some special characters of note: \n for new line, \t for tab. They are each preceded by \

  • Output may be nicely formatted using %

  • We can read input using raw_input()

What to work on before next class:

  • Go through all the exercises in the course notes 4 and 5.
  • Review how you can import modules to use. Import and use the math module.
  • There are many ways to write strings, so you should spend some time on learning how strings work before using them in programs.
  • Creating strings: Create strings in many different ways. Write a program that creates different strings and prints them
  • Strings with single quotes
  • Strings with double quotes
  • Strings with triple quotes
  • Strings with a new line in them
  • Strings with a backslash in it
  • Strings with three lines and three columns separated by tabs
>>> x = 'abc'
>>> y = "def"
>>> z = """ghi"""
>>> w = "def\\abc"
>>> x = 'abc\ndef'
>>> y = 'abc\tdef\nghi\tklm'

Now try:

>>> y
>>> print y

What is the difference?

Can you guess what the string will look like from the way it is written?

Remember the quotes must match.

  • String functions: Practice all the functions to make sure you understand how they work.
  • Concatenation (what is the output?)

    >>> 'abc' 'def'
    >>> 'abc' + 'def'
    >>> 'abc ' + 'def'
    >>> x = 'abc'
    >>> y = 'def'
    >>> x+y
    >>> x y
  • Replication

    >>> 'abc'*4
    >>> 'abc '*4
  • String length len()

    >>> x = 'abc'
    >>> len(x)
  • String conversion

    >>> x = str(3)
    >>> y = int('3')
    >>> z = float('3')
    >>> z = float('3.2')
    >>> w = 4.5
    >>> str(4.5)

    Make some mistakes and see what happens:

    >>> x = int('abc')
    >>> y = '3'
    >>> y+5
    >>> z = 'abc"
    >>> x = int('3.2')

    You see int() works differently for strings. The string must contain an integer for it to work.

  • Printing strings: now understand how to print strings

    >>> x = 'abc'
    >>> print x
    >>> x = "'abc'"
    >>> print x
    >>> x = """ "abc" """
    >>> print x
    >>> print 'This is lecture %d of Spring 2014'
    >>> print 'This is lecture %d of Spring 2014' %5
    >>> numlecture = 5
    >>> print 'This is lecture %d of Spring 2014' %numlecture
    >>> semester = 'Spring'
    >>> year = 2014
    >>> print 'This is lecture %d of %s %d' %numlecture
    >>> print 'This is lecture %d of %s %d' %(numlecture, semester, year)

    Compare with:

    >>> print 'This is lecture', numlecture, 'of', semester, year

    Every time you put a comma, a new space is added.

    >>> import math
    >>> print 'value of pi is', math.pi
    >>> print 'value of pi with 4 decimal points is %.4f' %math.pi
    >>> print 'value of pi with 2 decimal points is %.2f' %math.pi

    Note the values are rounded when you use formatting.

  • Reading input: Try the following and learn the difference

    >>> x = raw_input()
    >>> x
    >>> x = raw_input('Enter a value ==> ')
    >>> x
    >>> x = int(raw_input('Enter a value ==> '))
    >>> x

    And make some mistakes:

    >>> x = raw_input('Enter a value ==> ")
    >>> x = int(raw_input('Enter a value ==> ')

    See if you can spot the mistake before you run it.