Let's go over the basic data types that you will encounter in Python.
A number may be an integer, a real number, or a complex number. The usual operations are:
- addition and subtraction,
+and- - multiplication and division,
*and/ - power,
**
Here is an example:
2 ** (2 + 2) # 16 1j ** 2 # -1 1. + 3.0j
Note
The symbol for complex numbers
j is a symbol to denote the imaginary part of a complex number.
It is a syntactic element and should not be confused with multiplication by a variable. More on complex numbers can be found in section Numeric Types of Chapter 2, Variables and Basic Types.
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:
list(range(n))}creates a list withnelements, starting with zero:
print(list(range(5))) # returns [0, 1, 2, 3, 4]
lengives the length of a list:
len(['a', 1, 2, 34]) # returns 4
appendis used to append an element to a list:
L = ['a', 'b', 'c']
L[-1] # 'c'
L.append('d')
L # L is now ['a', 'b', 'c', 'd']
L[-1] # 'd'- The operator
+concatenates two lists:L1 = [1, 2] L2 = [3, 4] L = L1 + L2 # [1, 2, 3, 4] - As one might expect, multiplying a list with an integer concatenates the list with itself several times:
n*Lis equivalent to making n additions.L = [1, 2] 3 * L # [1, 2, 1, 2, 1, 2]
A Boolean expression is an expression that may have the value True or False. Some common operators that yield conditional expressions are as follow:
- Equal,
== - Not equal,
!= - Less than, Less than or equal to,
<,<= - Greater than, Greater than or equal to,
>,>=
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!