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 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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Find current directory and file's directory:
To get the full path to the directory a Python file is contained in, write this in that file:
import os dir_path = os.path.dirname(os.path.realpath(__file__))
(Note that the incantation above won't work if you've already used os.chdir() to change your current working directory, since the value of the __file__ constant is relative to the current working directory and is not changed by an os.chdir() call.)
To get the current working directory use
import os cwd = os.getcwd()
Documentation references for the modules, constants and functions used above:
- The os and os.path modules.
- The __file__ constant
- os.path.realpath(path) (returns "the canonical path of the specified filename, eliminating any symbolic links encountered in the path")
- os.path.dirname(path) (returns "the directory name of pathname path")
- os.getcwd() (returns "a string representing the current working directory")
- os.chdir(path) ("change the current working directory to path")
Ref: https://bit.ly/3fy0R6m
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