3.1.1. Numbers¶

The interpreter acts as a simple calculator: you can type an expression at it
and it will write the value. Expression syntax is straightforward: the
operators + , - , * and / work just like in most other languages
(for example, Pascal or C); parentheses ( () ) can be used for grouping.
For example:

>>> 2 + 2
4
>>> 50 - 5*6
20
>>> (50 - 5*6) / 4
5.0
>>> 8 / 5  # division always returns a floating point number
1.6

The integer numbers (e.g. 2 , 4 , 20 ) have type int ,
the ones with a fractional part (e.g. 5.0 , 1.6 ) have type
float . We will see more about numeric types later in the tutorial.

Division ( / ) always returns a float. To do floor division and
get an integer result you can use the // operator; to calculate
the remainder you can use % :

>>> 17 / 3  # classic division returns a float
5.666666666666667
>>>
>>> 17 // 3  # floor division discards the fractional part
5
>>> 17 % 3  # the % operator returns the remainder of the division
2
>>> 5 * 3 + 2  # floored quotient * divisor + remainder
17

With Python, it is possible to use the ** operator to calculate powers 1:

>>> 5 ** 2  # 5 squared
25
>>> 2 ** 7  # 2 to the power of 7
128

The equal sign ( = ) is used to assign a value to a variable. Afterwards, no
result is displayed before the next interactive prompt:

>>> width = 20
>>> height = 5 * 9
>>> width * height
900

If a variable is not “defined” (assigned a value), trying to use it will
give you an error:

>>> n  # try to access an undefined variable
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'n' is not defined

There is full support for floating point; operators with mixed type operands
convert the integer operand to floating point:

>>> 4 * 3.75 - 1
14.0

In interactive mode, the last printed expression is assigned to the variable
_ . This means that when you are using Python as a desk calculator, it is
somewhat easier to continue calculations, for example:

>>> tax = 12.5 / 100
>>> price = 100.50
>>> price * tax
12.5625
>>> price + _
113.0625
>>> round(_, 2)
113.06

This variable should be treated as read-only by the user. Don’t explicitly
assign a value to it — you would create an independent local variable with the
same name masking the built-in variable with its magic behavior.

In addition to int and float , Python supports other types of
numbers, such as Decimal and Fraction .
Python also has built-in support for complex numbers ,
and uses the j or J suffix to indicate the imaginary part
(e.g. 3+5j ).

3.1.2. Strings¶

Besides numbers, Python can also manipulate strings, which can be expressed
in several ways. They can be enclosed in single quotes ( '...' ) or
double quotes ( "..." ) with the same result 2. \ can be used
to escape quotes:

>>> 'spam eggs'  # single quotes
'spam eggs'
>>> 'doesn\'t'  # use \' to escape the single quote...
"doesn't"
>>> "doesn't"  # ...or use double quotes instead
"doesn't"
>>> '"Yes," they said.'
'"Yes," they said.'
>>> "\"Yes,\" they said."
'"Yes," they said.'
>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'

In the interactive interpreter, the output string is enclosed in quotes and
special characters are escaped with backslashes. While this might sometimes
look different from the input (the enclosing quotes could change), the two
strings are equivalent. The string is enclosed in double quotes if
the string contains a single quote and no double quotes, otherwise it is
enclosed in single quotes. The print() function produces a more
readable output, by omitting the enclosing quotes and by printing escaped
and special characters:

>>> '"Isn\'t," they said.'
'"Isn\'t," they said.'
>>> print('"Isn\'t," they said.')
"Isn't," they said.
>>> s = 'First line.\nSecond line.'  # \n means newline
>>> s  # without print(), \n is included in the output
'First line.\nSecond line.'
>>> print(s)  # with print(), \n produces a new line
First line.
Second line.

If you don’t want characters prefaced by \ to be interpreted as
special characters, you can use raw strings by adding an r before
the first quote:

>>> print('C:\some\name')  # here \n means newline!
C:\some
ame
>>> print(r'C:\some\name')  # note the r before the quote
C:\some\name

There is one subtle aspect to raw strings: a raw string may not end in
an odd number of \ characters; see
the FAQ entry for more information
and workarounds.

String literals can span multiple lines. One way is using triple-quotes:
"""...""" or '''...''' . End of lines are automatically
included in the string, but it’s possible to prevent this by adding a \ at
the end of the line. The following example:

print("""\
Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to
""")

produces the following output (note that the initial newline is not included):

Usage: thingy [OPTIONS]
     -h                        Display this usage message
     -H hostname               Hostname to connect to

Strings can be concatenated (glued together) with the + operator, and
repeated with * :

>>> # 3 times 'un', followed by 'ium'
>>> 3 * 'un' + 'ium'
'unununium'

Two or more string literals (i.e. the ones enclosed between quotes) next
to each other are automatically concatenated.

>>> 'Py' 'thon'
'Python'

This feature is particularly useful when you want to break long strings:

>>> text = ('Put several strings within parentheses '
...         'to have them joined together.')
>>> text
'Put several strings within parentheses to have them joined together.'

This only works with two literals though, not with variables or expressions:

>>> prefix = 'Py'
>>> prefix 'thon'  # can't concatenate a variable and a string literal
  File "<stdin>", line 1
    prefix 'thon'
           ^^^^^^
SyntaxError: invalid syntax
>>> ('un' * 3) 'ium'
  File "<stdin>", line 1
    ('un' * 3) 'ium'
               ^^^^^
SyntaxError: invalid syntax

If you want to concatenate variables or a variable and a literal, use + :

>>> prefix + 'thon'
'Python'

Strings can be indexed (subscripted), with the first character having index 0.
There is no separate character type; a character is simply a string of size
one:

>>> word = 'Python'
>>> word[0]  # character in position 0
'P'
>>> word[5]  # character in position 5
'n'

Indices may also be negative numbers, to start counting from the right:

>>> word[-1]  # last character
'n'
>>> word[-2]  # second-last character
'o'
>>> word[-6]
'P'

Note that since -0 is the same as 0, negative indices start from -1.

In addition to indexing, slicing is also supported. While indexing is used
to obtain individual characters, slicing allows you to obtain substring:

>>> word[0:2]  # characters from position 0 (included) to 2 (excluded)
'Py'
>>> word[2:5]  # characters from position 2 (included) to 5 (excluded)
'tho'

Slice indices have useful defaults; an omitted first index defaults to zero, an
omitted second index defaults to the size of the string being sliced.

>>> word[:2]   # character from the beginning to position 2 (excluded)
'Py'
>>> word[4:]   # characters from position 4 (included) to the end
'on'
>>> word[-2:]  # characters from the second-last (included) to the end
'on'

Note how the start is always included, and the end always excluded. This
makes sure that s[:i] + s[i:] is always equal to s :

>>> word[:2] + word[2:]
'Python'
>>> word[:4] + word[4:]
'Python'

One way to remember how slices work is to think of the indices as pointing
between characters, with the left edge of the first character numbered 0.
Then the right edge of the last character of a string of n characters has
index n , for example:

 +---+---+---+---+---+---+
 | P | y | t | h | o | n |
 +---+---+---+---+---+---+
 0   1   2   3   4   5   6
-6  -5  -4  -3  -2  -1

The first row of numbers gives the position of the indices 0…6 in the string;
the second row gives the corresponding negative indices. The slice from i to
j consists of all characters between the edges labeled i and j ,
respectively.

For non-negative indices, the length of a slice is the difference of the
indices, if both are within bounds. For example, the length of word[1:3] is
2.

Attempting to use an index that is too large will result in an error:

>>> word[42]  # the word only has 6 characters
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
IndexError: string index out of range

However, out of range slice indexes are handled gracefully when used for
slicing:

>>> word[4:42]
'on'
>>> word[42:]
''

Python strings cannot be changed — they are immutable .
Therefore, assigning to an indexed position in the string results in an error:

>>> word[0] = 'J'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment
>>> word[2:] = 'py'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'str' object does not support item assignment

If you need a different string, you should create a new one:

>>> 'J' + word[1:]
'Jython'
>>> word[:2] + 'py'
'Pypy'

The built-in function len() returns the length of a string:

>>> s = 'supercalifragilisticexpialidocious'
>>> len(s)
34

3.1.3. Lists¶

Python knows a number of compound data types, used to group together other
values. The most versatile is the list , which can be written as a list of
comma-separated values (items) between square brackets. Lists might contain
items of different types, but usually the items all have the same type.

>>> squares = [1, 4, 9, 16, 25]
>>> squares
[1, 4, 9, 16, 25]

Like strings (and all other built-in sequence types), lists can be
indexed and sliced:

>>> squares[0]  # indexing returns the item
1
>>> squares[-1]
25
>>> squares[-3:]  # slicing returns a new list
[9, 16, 25]

All slice operations return a new list containing the requested elements. This
means that the following slice returns a
shallow copy of the list:

>>> squares[:]
[1, 4, 9, 16, 25]

Lists also support operations like concatenation:

>>> squares + [36, 49, 64, 81, 100]
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

Unlike strings, which are immutable , lists are a mutable
type, i.e. it is possible to change their content:

>>> cubes = [1, 8, 27, 65, 125]  # something's wrong here
>>> 4 ** 3  # the cube of 4 is 64, not 65!
64
>>> cubes[3] = 64  # replace the wrong value
>>> cubes
[1, 8, 27, 64, 125]

You can also add new items at the end of the list, by using
the append() method (we will see more about methods later):

>>> cubes.append(216)  # add the cube of 6
>>> cubes.append(7 ** 3)  # and the cube of 7
>>> cubes
[1, 8, 27, 64, 125, 216, 343]

Assignment to slices is also possible, and this can even change the size of the
list or clear it entirely:

>>> letters = ['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> letters
['a', 'b', 'c', 'd', 'e', 'f', 'g']
>>> # replace some values
>>> letters[2:5] = ['C', 'D', 'E']
>>> letters
['a', 'b', 'C', 'D', 'E', 'f', 'g']
>>> # now remove them
>>> letters[2:5] = []
>>> letters
['a', 'b', 'f', 'g']
>>> # clear the list by replacing all the elements with an empty list
>>> letters[:] = []
>>> letters
[]

The built-in function len() also applies to lists:

>>> letters = ['a', 'b', 'c', 'd']
>>> len(letters)
4

It is possible to nest lists (create lists containing other lists), for
example:

>>> a = ['a', 'b', 'c']
>>> n = [1, 2, 3]
>>> x = [a, n]
>>> x
[['a', 'b', 'c'], [1, 2, 3]]
>>> x[0]
['a', 'b', 'c']
>>> x[0][1]
'b'