Basic types

A number may be an integer, a real number, or a complex number. The usual operations are:

Here is an example:

2 ** (2 + 2) # 16
1j ** 2 # -1
1. + 3.0j

Strings are sequences of characters, enclosed by simple or double quotes:

'valid string'
"string with double quotes"
"you shouldn't forget comments"
'these are double quotes: ".." '

You can also use triple quotes for strings that have multiple lines:

"""This is
 a long,
 long string"""

A variable is a reference to an object. An object may have several references. One uses the assignment operator = to assign a value to a variable:

x = [3, 4] # a list object is created
y = x # this object now has two labels: x and y
del x # we delete one of the labels
del y # both labels are removed: the object is deleted

The value of a variable can be displayed by the print function:

x = [3, 4] # a list object is created
print(x)

Lists are a very useful construction and one of the basic types in Python. A Python list is an ordered list of objects enclosed by square brackets. One can access the elements of a list using zero-based indexes inside square brackets:

L1 = [5, 6]
L1[0] # 5
L1[1] # 6
L1[2] # raises IndexError
L2 = ['a', 1, [3, 4]]
L2[0] # 'a'
L2[2][0] # 3
L2[-1] # last element: [3,4]
L2[-2] # second to last: 1

Indexing of the elements starts at zero. One can put objects of any type inside a list, even other lists. Some basic list functions are as follows:

      print(list(range(5))) # returns [0, 1, 2, 3, 4]

      len(['a', 1, 2, 34]) # returns 4

      L = ['a', 'b', 'c']
      L[-1] # 'c'
      L.append('d')
      L # L is now ['a', 'b', 'c', 'd']
      L[-1] # 'd'

A Boolean expression is an expression that may have the value True or False. Some common operators that yield conditional expressions are as follow:

One combines different Boolean values with or and and. The keyword not , gives the logical negation of the expression that follows. Comparisons can be chained so that, for example, x < y < z is equivalent to x < y and y < z. The difference is that y is only evaluated once in the first example. In both cases, z is not evaluated at all when the first condition, x < y, evaluates to False:

2 >= 4  # False
2 < 3 < 4 # True
2 < 3 and 3 < 2 # False
2 != 3 < 4 or False # True
2 <= 2 and 2 >= 2 # True
not 2 == 3 # True
not False or True and False # True!