1 - Declaration of variables in python
A variable is a kind of virtual box in which we can put one (or more) data (s). The idea is to temporarily store data to work with. For your machine a variable is an address that indicates the location of the RAM where the information we have linked is stored.
Python is a dynamically typed language, which means you do not have to declare the type of the variable. So to declare a variable in python, we directly introduce its name followed by its value:
Example
>>> x = 5
>>> # here we introduce a variable whose value is 5
>>> x
5
2 - Variable names and reserved words
Under Python, variable names must follow a few simple rules:
- A variable name is a sequence of letters a..z, A..Z and digits 0 .. 9, which must always begin with a letter.
- Only ordinary letters are allowed. Accented letters, cedillas, spaces, special characters such as $, #, @, etc. are prohibited except for the _ character.
- The case is significant (uppercase and lowercase characters are distinguished).
Warning ! : Phone, phone, PHONE are therefore different variables. Be careful !
In addition to these rules, it must be added that you can not use as variable names the 29 "reserved words" below (they are used by the Python language itself):
3 - Types of variables
In python a variable is dynamically typed, ie by introducing a variable followed by its value, Python detects its type:
Example
>>> h = "Hello"
>>> h
Hello
In this example Python detects that this is a string whose value is "Hello"
- Integer : as its name indicates an integer is a digit without decimals.
- Floats or number with commas: example: 1.5
- Strings or string of characters : to simplify everything that is not a number.
There are many others but, to simplify the task we will not mention all types here.
Note
To know the type of a variable, you can use the "type ()" function
Example
>>> x=5
>>> # Here we define a varible of Integer type
>>> type(x)
class int=""
>>> a="android phone"
>>> # Here we define a varible of String type
>>> type(a)
class str=""
>>> ram=4.5
>>> # Here we define a varible of float type
>>> type(ram)
class float=""
3 - Operations on the variables
Operators are symbols that manipulate variables, that is perform operations, evaluate them, ... There are several types of operators : calculation operators, assignment operators, comparison operators, logical operators, ...
3 - 1 Calculation operators
Calculation operators allow to mathematically modify the value of a variable :
- "+" addition operator Adds two values
- "-" subtraction operator Subtracts two values
- "*" multiplication operator Multiplies two values
- "/ " division operator Divides two values
- "=" assignment operator Assigns a value to a variable : x = 3 Put the value 3 in the variable x.
3 - 2 Assignment Operators
These operators simplify operations such as adding a value to a variable and storing the result in the variable. For example: x = x + 2
With the assignment operators it is possible to write this operation in the following form: x + = 2.
That is, if x = 3 it becomes after the operation 5.
3 - 3 Concatenation of variables
The concatenation of two strings variables is the operation of putting one of the strings followed by the other and storing the result in another variable
Example
>>> d="Desk"
>>> t="top"
>>> dt=d+t
>>> # Here dt is the concatenation of "Desk" and "top" and take as value "Desktop"
>>> dt
'Desktop'
Note : You can note concatenate a string variable with a numerical variable. But you can do it after transformation from the numeric type to the string type with the str() method.
Example
>>> ram=5.6
>>> print("The ram value is equal to : " + str(ram))
The ram value is equal to : 5.6
