1.1 variables

This lesson introduces data types and variables in python.

A variable is a way to link some data to a memory location. The memory here, does not mean the storage such as hard drive or USB etc. rather memory such as RAM. The memory size which is allocated for a variable depends on the kind and the amount of data linked to that variable. For example, a variable consisting 10 integers will hold less memory as compared to a variable consisting of a million integers. Similarly, a variable holding an integer (e.g. 92) will have different amount/size of memory as compared to a variable holding a string say Ali. When we define a variable in python, the python checks the type of the variable and allocates some memory for that variable.

a = 12

So what has been done above is that a variable named a is assigned a value of 12. Behind the scenes python created an object and the variable name a is a reference for that object.

a = 14

When the variable a is redefined, it means the location of memory which was holding the value 12 before, now holds 14. This means we have changed the contents of memory.

a = a + 12
print(a)

26

The print is a (builtin) function in python which we can use to print the value of a variable. This is not always true but more about it will come in 1.11 print and 3.16 magic methods. The kind of object python creates, depends upon the type of data. We can check the type of a variable by using the command/function type(VariableName)

print(type(a))

<class 'int'>

A function is a different creature in python. We will cover more about it later in 1.12 functions . At this point, just keep in mind that when some object is a function, we can call it by appending paranthesis () after its name Above we have called the print function. Don’t worry if you don’t understand the meaning of calling a function at this point.

The function type is the most important function in python. Whenver you don’t know about some object in python, the first thing you should do is to check its type using type(object)

b = 30.0
print(type(b))

<class 'float'>

Question

Both a and b contained the value thirty, then why they had different types?

c = a + b
print(c, type(c))

56.0 <class 'float'>

d = a + a
print(d, type(d))

52 <class 'int'>

It is important to note that the python is able to change the type of new variable based on the kind of value assigned to it. If a float value is assigned to a variable, python will change the type of this variable to float.

Another significant thing is that, we can assign any type of data to a variable. For example, we can assign int to variable a, later we can assign float to the variable a and then we can assign a completely different type like string to the variable a.

This is a blessing (in terms of ease of use) as well as a curse (in terms of its slow speed) of python and for python users.

a = 'Ali'
print(type(a))

<class 'str'>

Question

Find out 14 different types in python. We have already seen three types above i.e. str, int and float.

When we assign a value to a variable and then assign that variable to a new variable, then both of these variables actually refer to the same object. We can verify this using the function id(VariableName)

a = 12
b = a
id(a), id(b)

(139692357458576, 139692357458576)

Above we have checked the identity of both a and b. The identity of an object in python is the memory address of that object. It means the address in memory (RAM) where that objected is stored/put. Since both a and b refer to the same object, they have the same memory address. We can also say that since both a and b have the same memory address, that means they are same objects.

b = 14
id(a), id(b)

(139692357458576, 139692357458640)

So when we assigned a different data to b, a new object was created and now b refers to this new object and thus its identity (memory address) changes now.

Just as there are conventions for naming people in a society, there is convention and rules for naming variables in python. The name of a variable can be any alpha-numeric combination with some exceptions. Following are valid variable names

ali9 = 12
Ali9 = 14
아타르 = 2
print(아타르)

2

A variable name must not start from a number.

# uncomment following line
# 1_ali = 29

Question: Explain the error which will result from the above code.

We can not name certain keywords as variable names. These keywords can be seen official python docs website 1

Data Types

Data type signifies the type of operation that can be performed on that data. The three common data types in python are as follows

• numeric

• sequence

• boolean

Numbers

To represent numerical values, python has three types

• integer

• float

• complex

a = 1
b = 0b101   # binary with base 2
x = 0o14  # octal with base 8
y = 0xe     # hexadecimals with base 16

print(a, type(a))
print(b, type(b))
print(x, type(x))
print(y, type(y))

1 <class 'int'>
5 <class 'int'>
12 <class 'int'>
14 <class 'int'>

0b101, 0o14 and 0xe are examples of integers represented in binary, octal and hexadecimal bases respectively. We can convert a number to binary, octal and hexadecimal using the functions bin, oct and hex respectively as follows

print(bin(5))
print(oct(12))
print(hex(14))

0b101
0o14
0xe

However, if you don’t understand the meaning of binary, octal and hexadecimal bases, don’t worry. This is not important at this point. Just keep in mind that we can represent numbers in different bases in python.

a = 12.5e3
print(type(a))

<class 'float'>

Since the value of a is a float, the type of a is float. The value of a is 12.5 times 10 raised to the power 3. This is called scientific notation. We can also represent a number in scientific notation as follows

a = 12.5e-3

Question What is the value of a in the above code?

Question:

Define variables to store the information on your ID card and print each of them. The variables should not be of same type.

x = 3 + 4j  # consist of real and imaginary part
print(type(x))

<class 'complex'>

coke = False
water = True
print(type(coke))

<class 'bool'>

Question What will the suitable data type to store currency values? Explain your reasoning.

Sequence

boolean

1

https://docs.python.org/2.5/ref/keywords.html