Author: SUNNY BHAVEEN CHANDRA

For more information -

• [1] Python notes- https://c17hawke.github.io/Python/

• [2] Python YouTube Playlist- https://youtube.com/playlist?list=PLrdaCCBhU_hnxIzB7EJlY-pfYOMGRycAK

Variables 🔤¶

• In programming languages, a variable is a name used to identify a value.
• Developers use variables to make their code more readable and understandable, like using names in natural language.
• Technically speaking, variables in Python are values that can change during the execution of a program.
• A variable is used to store a value that may change as needed.
• In Python, there is no need to explicitly declare a variable. As soon as you assign a value to it, the variable is considered declared.

Define a Variable 🔍-¶

In Python, a variable is defined by assigning a value to it. Unlike some other programming languages, there is no need to declare a variable before assigning a value to it.

In [1]:

# let x be 100

x = 100
print(x)

# let x be 100 x = 100 print(x)

100

In [2]:

x = 5
print("Twice of a num", x * 2)

x = 5 print("Twice of a num", x * 2)

Twice of a num 10

In [3]:

lecture = "Introduction to python"
print("This is a lecture of:", lecture)

lecture = "Introduction to python" print("This is a lecture of:", lecture)

This is a lecture of: Introduction to python

NOTE: f-string way to write above statement - (Most frequently used)

In [4]:

print(f"This is a lecture of: {lecture}") # do not forget f in the beginning

print(f"This is a lecture of: {lecture}") # do not forget f in the beginning

This is a lecture of: Introduction to python

In [5]:

name = "Sunny"
print(f"My name is: {name}")
age = 52
print(f"My age is: {age}")
print(f"My name is {name} and My age is {age} years")

name = "Sunny" print(f"My name is: {name}") age = 52 print(f"My age is: {age}") print(f"My name is {name} and My age is {age} years")

My name is: Sunny
My age is: 52
My name is Sunny and My age is 52 years

In [6]:

# write a table of a no. upto 10
x = 5
print(f"showing table of: {x}")
print(f"{x} x 1  = {x * 1}")
print(f"{x} x 2  = {x * 2}")
print(f"{x} x 3  = {x * 3}")
print(f"{x} x 4  = {x * 4}")
print(f"{x} x 5  = {x * 5}")
print(f"{x} x 6  = {x * 6}")
print(f"{x} x 7  = {x * 7}")
print(f"{x} x 8  = {x * 8}")
print(f"{x} x 9  = {x * 9}")
print(f"{x} x 10 = {x * 10}")

# write a table of a no. upto 10 x = 5 print(f"showing table of: {x}") print(f"{x} x 1 = {x * 1}") print(f"{x} x 2 = {x * 2}") print(f"{x} x 3 = {x * 3}") print(f"{x} x 4 = {x * 4}") print(f"{x} x 5 = {x * 5}") print(f"{x} x 6 = {x * 6}") print(f"{x} x 7 = {x * 7}") print(f"{x} x 8 = {x * 8}") print(f"{x} x 9 = {x * 9}") print(f"{x} x 10 = {x * 10}")

showing table of: 5
5 x 1  = 5
5 x 2  = 10
5 x 3  = 15
5 x 4  = 20
5 x 5  = 25
5 x 6  = 30
5 x 7  = 35
5 x 8  = 40
5 x 9  = 45
5 x 10 = 50

Rules for defining the variables in Python 📏-¶

1. Variable names can contain letters, numbers, and underscores only (A-Z, a-z, 0-9, and _ ), but cannot start with a number.

In [7]:

# Correct variable names as per above rule
my_variable = 10
my_variable_1 = 20
MyVariable_1 = 20
_variable = 200

# Correct variable names as per above rule my_variable = 10 my_variable_1 = 20 MyVariable_1 = 20 _variable = 200

In [8]:

# against the rule above
1_variable = 100 # This will raise a SyntaxError

# against the rule above 1_variable = 100 # This will raise a SyntaxError

  Cell In[8], line 2
    1_variable = 100 # This will raise a SyntaxError
     ^
SyntaxError: invalid decimal literal

In [9]:

my_variable_@ = 20 # This will raise a SyntaxError

my_variable_@ = 20 # This will raise a SyntaxError

  Cell In[9], line 1
    my_variable_@ = 20 # This will raise a SyntaxError
                  ^
SyntaxError: invalid syntax

2. Variable names are case-sensitive.

In [10]:

# These are two different variables
my_variable = 10
my_Variable = 20

print(my_variable)
print(my_Variable)

# These are two different variables my_variable = 10 my_Variable = 20 print(my_variable) print(my_Variable)

10
20

3. Variable names cannot be the same as Python keywords or inbuilt function.

NOTE: Think of keywords as a vocabulary of python language.

In [11]:

# You can check the list of Python keywords using the keyword module
import keyword
print(keyword.kwlist)

# You can check the list of Python keywords using the keyword module import keyword print(keyword.kwlist)

['False', 'None', 'True', '__peg_parser__', 'and', 'as', 'assert', 'async', 'await', 'break', 'class', 'continue', 'def', 'del', 'elif', 'else', 'except', 'finally', 'for', 'from', 'global', 'if', 'import', 'in', 'is', 'lambda', 'nonlocal', 'not', 'or', 'pass', 'raise', 'return', 'try', 'while', 'with', 'yield']

In [12]:

# to print the builtins present in python
import builtins
print(dir(builtins))

# to print the builtins present in python import builtins print(dir(builtins))

['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', 'BlockingIOError', 'BrokenPipeError', 'BufferError', 'BytesWarning', 'ChildProcessError', 'ConnectionAbortedError', 'ConnectionError', 'ConnectionRefusedError', 'ConnectionResetError', 'DeprecationWarning', 'EOFError', 'Ellipsis', 'EnvironmentError', 'Exception', 'False', 'FileExistsError', 'FileNotFoundError', 'FloatingPointError', 'FutureWarning', 'GeneratorExit', 'IOError', 'ImportError', 'ImportWarning', 'IndentationError', 'IndexError', 'InterruptedError', 'IsADirectoryError', 'KeyError', 'KeyboardInterrupt', 'LookupError', 'MemoryError', 'ModuleNotFoundError', 'NameError', 'None', 'NotADirectoryError', 'NotImplemented', 'NotImplementedError', 'OSError', 'OverflowError', 'PendingDeprecationWarning', 'PermissionError', 'ProcessLookupError', 'RecursionError', 'ReferenceError', 'ResourceWarning', 'RuntimeError', 'RuntimeWarning', 'StopAsyncIteration', 'StopIteration', 'SyntaxError', 'SyntaxWarning', 'SystemError', 'SystemExit', 'TabError', 'TimeoutError', 'True', 'TypeError', 'UnboundLocalError', 'UnicodeDecodeError', 'UnicodeEncodeError', 'UnicodeError', 'UnicodeTranslateError', 'UnicodeWarning', 'UserWarning', 'ValueError', 'Warning', 'WindowsError', 'ZeroDivisionError', '__IPYTHON__', '__build_class__', '__debug__', '__doc__', '__import__', '__loader__', '__name__', '__package__', '__spec__', 'abs', 'all', 'any', 'ascii', 'bin', 'bool', 'breakpoint', 'bytearray', 'bytes', 'callable', 'chr', 'classmethod', 'compile', 'complex', 'copyright', 'credits', 'delattr', 'dict', 'dir', 'display', 'divmod', 'enumerate', 'eval', 'exec', 'execfile', 'filter', 'float', 'format', 'frozenset', 'get_ipython', 'getattr', 'globals', 'hasattr', 'hash', 'help', 'hex', 'id', 'input', 'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'range', 'repr', 'reversed', 'round', 'runfile', 'set', 'setattr', 'slice', 'sorted', 'staticmethod', 'str', 'sum', 'super', 'tuple', 'type', 'vars', 'zip']

In [13]:

# against the above rule
print = 12
print(print) # type error

# against the above rule print = 12 print(print) # type error

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[13], line 3
      1 # against the above rule
      2 print = 12
----> 3 print(print) # type error

TypeError: 'int' object is not callable

In [14]:

# Invalid variable name
for = 10 # This will raise a SyntaxError

# Invalid variable name for = 10 # This will raise a SyntaxError

  Cell In[14], line 2
    for = 10 # This will raise a SyntaxError
        ^
SyntaxError: invalid syntax

In [15]:

# ! WARNING: Only use in Jupyter notebooks -
del print # deleting the instance of print in jupyter notebook only

# ! WARNING: Only use in Jupyter notebooks - del print # deleting the instance of print in jupyter notebook only

In [16]:

p = 10
print(p) # Now it works

p = 10 print(p) # Now it works

10

Conventions 📝¶

1. following is an example of declaring a hidden variable

In [17]:

_variable = 200 # it gets grayed out as by some IDEs

_variable = 200 # it gets grayed out as by some IDEs

2. Generally names of variables should start with small letters

In [18]:

variable = 200
x_val = 100

variable = 200 x_val = 100

3. Name of constants or the values that you do not wish to change in the program are constants

In [19]:

PI = 3.14
radius = 3
area_of_circle = PI * radius * radius
# area_of_circle = PI * radius ** 2 # same as above 
print(f"Area of circle with radius: {radius} is {area_of_circle} unit-square")

PI = 3.14 radius = 3 area_of_circle = PI * radius * radius # area_of_circle = PI * radius ** 2 # same as above print(f"Area of circle with radius: {radius} is {area_of_circle} unit-square")

Area of circle with radius: 3 is 28.259999999999998 unit-square

Features 🔍¶

1. There is no need to declare a variable before assigning a value to it.

example in other languages like C or C++

int x = 2;

In [20]:

# You can directly assign a value to a variable
x = 2
y = "sample text"

# You can directly assign a value to a variable x = 2 y = "sample text"

In [21]:

# advance - TYPE hinting
x: int = 2
y: str = "example text"

# advance - TYPE hinting x: int = 2 y: str = "example text"

2. You can assign values to multiple variables in one line.

In [22]:

x, y, z = 10, 20, 30

print(x, y, z)

# This is equivalent to:
x = 10
y = 20
z = 30

print(x, y, z)

x, y, z = 10, 20, 30 print(x, y, z) # This is equivalent to: x = 10 y = 20 z = 30 print(x, y, z)

10 20 30
10 20 30

More real world examples 🌎¶

In [23]:

length = 10
width = 20

area_of_rectangle = length * width
perimeter = 2 * (length + width)

print(f"for rectangle with length: {length} and width: {width}")
print(f"area of rectangle = {area_of_rectangle}")
print(f"Perimeter of rectangle = {perimeter}")

length = 10 width = 20 area_of_rectangle = length * width perimeter = 2 * (length + width) print(f"for rectangle with length: {length} and width: {width}") print(f"area of rectangle = {area_of_rectangle}") print(f"Perimeter of rectangle = {perimeter}")

for rectangle with length: 10 and width: 20
area of rectangle = 200
Perimeter of rectangle = 60

In [24]:

# More real world examples -

cost_of_shirt = 600
units_to_purchase = 3

amount_payable = cost_of_shirt * units_to_purchase
print(f"Cost of 1 shirt: {cost_of_shirt}")
print(f"Units to purchase: {units_to_purchase}")
print(f"Amount payable (in INR): {amount_payable}")

# More real world examples - cost_of_shirt = 600 units_to_purchase = 3 amount_payable = cost_of_shirt * units_to_purchase print(f"Cost of 1 shirt: {cost_of_shirt}") print(f"Units to purchase: {units_to_purchase}") print(f"Amount payable (in INR): {amount_payable}")

Cost of 1 shirt: 600
Units to purchase: 3
Amount payable (in INR): 1800