String Literal

A string literal is a sequence of characters. In Python, this type is called str. Strings in Python start and end with a single quotes (') or double quotes ("). A string can be made up of letters, numbers, and special characters. For example:

>>> 'hello'
'hello'
>>> 'how are you?'
'how are you?'
>>> 'short- and long-term'
'short- and long-term'

If a string begins with a single quote, it must end with a single quote. The same applies to double-quoted strings. You can not mix the type of quotes.

Escape Sequences

To include a quote within a string, use an escape character (\) before it. Otherwise Python interprets that quote as the end of a string and an error occurs. For example, the following code results in an error because Python does not expect anything to come after the second quote:

>>> storm_greeting = 'wow, you're dripping wet.'
SyntaxError: invalid syntax
The escape sequence \' indicates that the second quote is simply a quote, not the end of the string: 
>>> storm_greeting = 'Wow, you\'re dripping wet.'
"Wow, you're dripping wet."

An alternative approach is to use a double-quoted string when including a single-quote within it, or vice-versa. Single- and double-quoted strings are equivalent. For example, when we used double-quotes to indicate the beginning and end of the string, the single-quote in you're no longer causes an error:

>>> storm_greeting = "Wow, you're dripping wet."
"Wow, you're dripping wet."

String Operators

Expression Description Example Output
str1 + str2 concatenate str1 and str1 print('ab' + 'c') abc
str1 * int1 concatenate int1 copies of str1 print('a' * 5) aaaaa
int1 * str1 concatenate int1 copies of str1 print(4 * 'bc') bcbcbcbc

Note: concatenate means to join together

The * and + operands obey by the standard precedence rules when used with strings.

All other mathematical operators and operands result in a TypeError.

Function print

Python has a built-in function named print that displays messages to the user. For example, the following function call displays the string "hello":

>>> print("hello")
hello
In the output above, notice that hello is displayed without the quotation marks. The quotes are only for Python's internal string formatting and are not seen by the user.

The print function may also be called with a mathematical expression for an argument. Python evaluates the mathematical expression first and then displays the resulting value to the user. For example:

>>> print(3 + 7 - 3)
7

Finally, print can take in more than one argument. Each pair of arguments is separated by a comma and a space is inserted between them when they are displayed. For example:

>>> print("hello", "there")
hello there

return vs. print

Recall: The general form of a return statement: 
return expression
When a return statement executes, the expression is evaluated to produce a memory address.
  • What is passed back to the caller?
    That memory address is passed back to the caller.
  • What is displayed?
    Nothing!

An example of return:

>>> def square_return(num):
    return num ** 2
>>> answer_return = square_return(4)  
>>> answer_return 
16

The general form of a print function call:

print(arguments)
When a print function call is executed, the argument(s) are evaluated to produce memory address(es).
  • What is passed back to the caller?
    Nothing!
  • What is displayed?
    The values at those memory address(es) are displayed on the screen.

An example of print:

>>> def square_print(num):
    print("The square of num is", num ** 2)
>>> answer_print = square_print(4)
The square num is 16
>>> answer_print
>>>

Function input

The function input is a built-in function that prompts the user to enter some input. The program waits for the user to enter the input, before executing the subsequent instructions. The value returned from this function is always a string. For example:

>>> input("What is your name? ")
What is your name? Jen
'Jen'
>>> name = input("What is your name? ")
What is your name? Jen
>>> name
'Jen'
>>> location = input("What is your location? ")
What is your location? Toronto
>>> location
'Toronto'
>>> print(name, "lives in", location)
Jen lives in Toronto
>>> num_coffee = input("How many cups of coffee? ")
How many cups of coffee? 2
>>> num_coffee
'2'

Operations on strings

Operation Description Example Output
str1 + str2 concatenate str1 and str1 print('ab' + 'c') abc
str1 * int1 concatenate int1 copies of str1 print('a' * 5) aaaaa
int1 * str1 concatenate int1 copies of str1 print(4 * 'bc') bcbcbcbc

Triple-quoted strings

We have used single- and double- quotes to represent strings. The third string format uses triple-quotes and a triple-quoted string can span multiple lines. For example:

>>> print(''' How
are
you?''')
How
are 
you?

Escape Sequences

Python has a special character called an escape character: \. When the escape character is used in a string, the character following the escape character is treated differently from normal. The escape character together with the character that follows it is an escape sequence. The table below contains some of Python's commonly used escape sequences.

Escape Sequence Name Example Output
\n newline (ASCII linefeed - LF) print('''How
are
you?''')		 How
are
you?
\t tab (ASCII horizontal tab - TAB) print('3\t4\t5') 
3   4       5
\\ backslash (\) 
print('\\')

\
\' single quote (') 
print('don\'t')

don't
\" double quote (") 
print("He says, \"hi\".")

He says, "hi".

Built-in function help displays the docstring from a function definition. For example, consider this function:

def area(base, height):
    """(number, number) -> number

    Return the area of a triangle with dimensions base
    and height.
    """

    return base * height / 2

Calling help on function area produces this output:

>>> help(area)
Help on function area in module __main__:

area(base, height)
    (number, number) -> number
    
    Return the area of a triangle with dimensions base
    and height.

The Six Steps

  1. Examples
    • What should your function do?
    • Type a couple of example calls.
    • Pick a name (often a verb or verb phrase): What is a short answer to "What does your function do"?
  2. Type Contract
    • What are the parameter types?
    • What type of value is returned?
  3. Header
    • Pick meaningful parameter names.
  4. Description
    • Mention every parameter in your description.
    • Describe the return value.
  5. Body
    • Write the body of your function.
  6. Test
    • Run the examples.

Applying the Design Recipe

The problem:

The United States measures temperature in Fahrenheit and Canada measures it in Celsius. When travelling between the two countries it helps to have a conversion function. Write a function that converts from Fahrenheit to Celsius.

  1. Examples 
        >>> convert_to_celsius(32)
    0
    >>> convert_to_celsius(212)
    100
  2. Type Contract 
        (number) -> number
  3. Header 
        def convert_to_celsius(fahrenheit):
  4. Description 
        Return the number of Celsius degrees equivalent to fahrenheit degrees.
  5. Body 
            return (fahrenheit - 32) * 5 / 9
  6. Test 
         Run the examples.

Putting it all together:

def convert_to_celsius(fahrenheit):
   ''' (number) -> number
   
   Return the number of Celsius degrees equivalent to fahrenheit degrees.
   
   >>> convert_to_ccelsius(32)
   0
   >>> convert_to_celsius(212)
   100
   '''
   
   return (fahrenheit - 32) * 5 / 9