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Python Variable

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Python Variable

Variable

Variables: containers for storing data values.

Creating Variables

Python has no command for declaring a variable.
A variable is created the moment you first assign a value to it.
Variables do not need to be declared with any particular type, and can even change type after they have been set.

x = 4       # x is of type int
x = "Sally" # x is now of type str
print(x)

Declare multiple variables in one line

Python allows you to assign values to multiple variables in one line.
- Note: Make sure the number of variables matches the number of values, or else you will get an error.

x, y, z = "Orange", "Banana", "Cherry"
print(x)    # Orange
print(y)    # Banana
print(z)    # Cherry

And you can assign the same value to multiple variables in one line.

x = y = z = "Orange"
print(x)    # Orange
print(y)    # Orange
print(z)    # Orange

Unpack a Collection

unpacking: to extract the values in a collection, such as list, tuple etc, into variables.

fruits = ["apple", "banana", "cherry"]
x, y, z = fruits
print(x)    # apple
print(y)    # banana
print(z)    # cherry

Variable Names

A variable can have a short name (like x and y) or a more descriptive name (age, carname, total*volume).
Rules for Python variables:
- A variable name must start with a letter or the underscore character
- A variable name cannot start with a number
- A variable name can only contain alpha-numeric characters and underscores (A-z, 0-9, and _ )
- Variable names are case-sensitive (age, Age and AGE are three different variables)
- A variable name cannot be any of the Python keywords.
Camel Case: Each word, except the first, starts with a capital letter.
Pascal Case: Each word starts with a capital letter.
Snake Case: Each word is separated by an underscore character.

myvar = "John"
my_var = "John"
_my_var = "John"
myVar = "John"
MYVAR = "John"
myvar2 = "John"

# illegal variable name
# 2myvar = "John"
# my-var = "John"
# my var = "John"

# Camel Case
myVariableName = "John"
# Pascal Case
MyVariableName = "John"
# Snake Case
my_variable_name = "John"

Scope

Output Variables

The Python print() function is often used to output variables.
- output multiple variables, separated by a comma.
- use the + operator to output multiple variables
The best way to output multiple variables in the print() function is to separate them with commas, which even support different data types.

x = "Python"
y = "is"
z = "awesome"
print(x, y, z)  # Python is awesome
print(x + y + z)    # Pythonisawesome

x = 5
y = 10
print(x + y)    # 15

x = 5
y = "John"
print(x + y)    #error: combine a string and a number with the + operator, Python will give you an error.
print(x, y)     #5 John

Global Variables

global variables: Variables that are created outside of a function.
local variables: variables only used within a function.
The global variable with the same name will remain as it was, global and with the original value.

x = "awesome"   # global variable

def myfunc():
  x = "fantastic"   # local variable
  print("Python is " + x)   # Python is fantastic

myfunc()

print("Python is " + x)     # Python is awesome

global Keyword
- To create a global variable inside a function
- to change a global variable inside a function.

# create a global variable
def myfunc():
  global x  # declare x inside myfunc() as a global variable
  x = "fantastic"

myfunc()

print("Python is " + x) # Python is fantastic

# change a global variable
x = "awesome"

def myfunc():
  global x  # declare x inside myfunc() as a global variable
  x = "fantastic"

myfunc()

print("Python is " + x)  # Python is fantastic

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