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Python Variable

Variable and Value

  • A variable is a memory location where a programmer can store a value. Example : roll_no, amount, name etc.
  • Value is either string, numeric etc. Example : "Sara", 120, 25.36
  • Variables are created when first assigned.
  • Variables must be assigned before being referenced.
  • The value stored in a variable can be accessed or updated later.
  • No declaration required
  • The type (string, int, float etc.) of the variable is determined by Python
  • The interpreter allocates memory on the basis of the data type of a variable.
Python Variable

Python Variable Name Rules

  • Must begin with a letter (a - z, A - B) or underscore (_)
  • Other characters can be letters, numbers or _
  • Case Sensitive
  • Can be any (reasonable) length
  • There are some reserved words which you cannot use as a variable name because Python uses them for other things.

Good Variable Name

  • Choose meaningful name instead of short name. roll_no is better than rn.
  • Maintain the length of a variable name. Roll_no_of_a-student is too long?
  • Be consistent; roll_no or RollNo
  • Begin a variable name with an underscore(_) character for a special case.

Python Assignment Statements

The assignment statement creates new variables and gives them values. Basic assignment statement in Python is :

Syntax:

<variable> = <expr>

Where the equal sign (=) is used to assign value (right side) to a variable name (left side). See the following statements :

>>> Item_name = "Computer" #A String
>>> Item_qty = 10 #An Integer
>>> Item_value = 1000.23 #A floating point
>>> print(Item_name)
Computer
>>> print(Item_qty)
10
>>> print(Item_value)
1000.23
>>> 

One thing is important, assignment statement read right to left only.

Example:

a = 12 is correct, but 12 = a does not make sense to Python, which creates a syntax error. Check it in Python Shell.

>>> a = 12
>>> 12 = a
SyntaxError: can't assign to literal
>>> 

Multiple Assignment

The basic assignment statement works for a single variable and a single expression. You can also assign a single value to more than one variables simultaneously.

Syntax:

var1=var2=var3...varn= = <expr>

Example:

x = y = z = 1 

Now check the individual value in Python Shell.

>>> x = y = z = 1
>>> print(x)
1
>>> print(y)
1
>>> print(z)
1
>>> 

Here is an another assignment statement where the variables assign many values at the same time.

Syntax:

<var>, <var>, ..., <var> = <expr>, <expr>, ..., <expr>

Example:

x, y, z = 1, 2, "abcd"

In the above example x, y and z simultaneously get the new values 1, 2 and "abcd".

>>> x,y,z = 1,2,"abcd"
>>> print(x)
1
>>> print(y)
2
>>> print(z)
abcd 

You can reuse variable names by simply assigning a new value to them :

>>> x = 100
>>> print(x)
100
>>> x = "Python"
>>> print(x)
Python
>>>  

Other ways to define value

>>> five_millions = 5_000_000
>>> five_millions

Output:

5000000
>>> small_int = .35
>>> small_int

Output:

0.35
>>> c_thousand = 10e3
>>> c_thousand

Output:

10000.0

Swap variables

Python swap values in a single line and this applies to all objects in python.

Syntax:

var1, var2 = var2, var1

Example:

>>> x = 10
>>> y = 20
>>> print(x)
10
>>> print(y)
20
>>> x, y = y, x
>>> print(x)
20
>>> print(y)
10
>>>

Local and global variables in Python

In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

Example:


var1 = "Python"
def func1():
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  Python

You can use a global variable in other functions by declaring it as global keyword :

Example:


def func1():
    global var1
    var1 = "PHP"
    print("In side func1() var1 = ",var1)

def func2():
    print("In side func2() var1 = ",var1)
func1()
func2()

Output:

In side func1() var1 =  PHP
In side func2() var1 =  PHP

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Python: Tips of the Day

Python: Time library

Time library provides lots of time related functions and methods and is good to know whether you're developing a website or apps and games or working with data science or trading financial markets. Time is essential in most development pursuits and Python's standard time library comes very handy for that.

Let's check out a few simple examples:

moment=time.strftime("%Y-%b-%d__%H_%M_%S",time.localtime())

import time
time_now=time.strftime("%H:%M:%S",time.localtime())
print(time_now)
date_now=time.strftime("%Y-%b-%d",time.localtime())
print(date_now)

Output:

11:36:34
2020-Nov-30