Python if...else – Mastering Decision Making

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Tags:- Python

Control flow is essential in any programming language. Python uses if, elif, and else statements to make decisions in your programs based on conditions.

In this tutorial, you’ll learn:

  • The basic structure of if, elif, and else

  • How conditions work

  • Nested and chained conditions

  • Common mistakes

  • Tips and tricks

  • A complete example at the end


What Is if...else?

Python's if...else statements let you run code based on whether a condition is true or false.

✅ Basic Structure

if condition:
    # code to run if condition is true
elif another_condition:
    # code to run if another_condition is true
else:
    # code to run if no conditions are true

Example 1: Simple if Statement

x = 10
if x > 5:
    print("x is greater than 5")

Output:

x is greater than 5

Example 2: if...else Statement

x = 3
if x > 5:
    print("x is greater than 5")
else:
    print("x is 5 or less")

Example 3: if...elif...else

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: F")

Understanding Boolean Conditions

Python evaluates conditions as either True or False.

# Comparisons
x == 5     # True if x equals 5
x != 10    # True if x does not equal 10
x > 3      # Greater than
x < 7      # Less than
x >= 3     # Greater than or equal
x <= 9     # Less than or equal

# Logical operators
x > 3 and x < 7     # True if both are True
x == 5 or x == 10   # True if either is True
not x == 5          # True if x is NOT 5

Nested if Statements

You can place an if inside another if:

age = 20
has_id = True

if age >= 18:
    if has_id:
        print("Access granted.")
    else:
        print("ID required.")
else:
    print("Access denied.")

Chained Conditions

You can use logical operators (and, or, not) to write complex conditions in a single line.

x = 25
if x > 10 and x < 50:
    print("x is between 10 and 50")

One-Liner if...else (Ternary Operator)

You can write simple if...else expressions in one line:

age = 18
status = "Adult" if age >= 18 else "Minor"
print(status)

Common Pitfalls

Pitfall Why It Happens Solution
Using = instead of == = is assignment, not comparison Use == in conditions
Missing colons (:) SyntaxError Always add : after condition
Indentation Errors Python uses indentation for blocks Use consistent 4-space indent
Overlapping Conditions Conditions may overlap Order matters; use elif wisely
Overusing Nested ifs Reduces readability Use logical operators instead

Tips

  • Use elif to avoid unnecessary nesting.

  • Always cover all logical cases with else as a fallback.

  • Use parentheses () for clarity in complex conditions.

  • Use comments to explain condition logic for beginners.

  • Test edge cases (e.g., ==, >=, <=) to avoid logic bugs.


Complete Example: Voting Eligibility

age = int(input("Enter your age: "))
citizen = input("Are you a citizen? (yes/no): ").lower()

if age >= 18:
    if citizen == "yes":
        print("You are eligible to vote.")
    else:
        print("You must be a citizen to vote.")
else:
    print("You must be at least 18 years old to vote.")

What’s Next?

After mastering if...else, explore:

  • While and for loops

  • Logical operators in depth

  • Exception handling with try...except

  • Functions to modularize your logic


Practice Challenge

Task: Write a program that checks if a number is positive, negative, or zero.

number = float(input("Enter a number: "))

if number > 0:
    print("Positive")
elif number < 0:
    print("Negative")
else:
    print("Zero")

 

Tips and Tricks


What is pass in Python?

Python | Pass Statement

The pass statement is used as a placeholder for future code. It represents a null operation in Python. It is generally used for the purpose of filling up empty blocks of code which may execute during runtime but has yet to be written.

 

def myfunction():
    pass

 


How can you generate random numbers?

Python | Generate random numbers

Python provides a module called random using which we can generate random numbers. e.g: print(random.random())

 

 

We have to import a random module and call the random() method as shown below:

 import random

 print(random.random())

The random() method generates float values lying between 0 and 1 randomly.


To generate customized random numbers between specified ranges, we can use the randrange() method
Syntax: randrange(beginning, end, step)
 

import random

print(random.randrange(5,100,2))

 


What is lambda in Python?

Python | Lambda function

A lambda function is a small anonymous function. This function can have any number of parameters but, can have just one statement.
 

 

Syntex: 
lambda arguments : expression
 

a = lambda x,y : x+y

print(a(5, 6))

It also provides a nice way to write closures. With that power, you can do things like this.

def adder(x):
    return lambda y: x + y

add5 = adder(5)

add5(1)    #6

As you can see from the snippet of Python, the function adder takes in an argument x and returns an anonymous function, or lambda, that takes another argument y. That anonymous function allows you to create functions from functions. This is a simple example, but it should convey the power lambdas and closures have.
 


What is swapcase() function in the Python?

Python | swapcase() Function

It is a string's function that converts all uppercase characters into lowercase and vice versa. It automatically ignores all the non-alphabetic characters.
 

string = "IT IS IN LOWERCASE."  

print(string.swapcase())  

 


How to remove whitespaces from a string in Python?

Python | strip() Function | Remove whitespaces from a string 

To remove the whitespaces and trailing spaces from the string, Python provides a strip([str]) built-in function. This function returns a copy of the string after removing whitespaces if present. Otherwise returns the original string.
 

string = "  Python " 
 
print(string.strip())  

 


What is the usage of enumerate() function in Python?

Python | enumerate() Function

The enumerate() function is used to iterate through the sequence and retrieve the index position and its corresponding value at the same time.
 

lst = ["A","B","C"] 
 
print (list(enumerate(lst)))

#[(0, 'A'), (1, 'B'), (2, 'C')]

 


Can you explain the filter(), map(), and reduce() functions?

Python | filter(), map(), and reduce() Functions

  • filter()  function accepts two arguments, a function and an iterable, where each element of the iterable is filtered through the function to test if the item is accepted or not.
    >>> set(filter(lambda x:x>4, range(7)))
    
    # {5, 6}
    
    

     

  • map() function calls the specified function for each item of an iterable and returns a list of result

    >>> set(map(lambda x:x**3, range(7)))
    
    # {0, 1, 64, 8, 216, 27, 125}

     

  • reduce() function reduces a sequence pair-wise, repeatedly until we arrive at a single value..
     

    >>> reduce(lambda x,y:y-x, [1,2,3,4,5])
    
    # 3
    

    Let’s understand this:

    2-1=1
    3-1=2
    4-2=2
    5-2=3

    Hence, 3.

 


What is a namedtuple?

Python | namedtuple

A namedtuple will let us access a tuple’s elements using a name/label. We use the function namedtuple() for this, and import it from collections.

>>> from collections import namedtuple

#format
>>> result=namedtuple('result','Physics Chemistry Maths') 

#declaring the tuple
>>> Chris=result(Physics=86,Chemistry=92,Maths=80) 

>>> Chris.Chemistry
# 92

 


Write a code to add the values of same keys in two different dictionaries and return a new dictionary.

We can use the Counter method from the collections module

from collections import Counter

dict1 = {'a': 5, 'b': 3, 'c': 2}
dict2 = {'a': 2, 'b': 4, 'c': 3}

new_dict = Counter(dict1) + Counter(dict2)


print(new_dict)
# Print: Counter({'a': 7, 'b': 7, 'c': 5})


 


Python In-place swapping of two numbers

 Python | In-place swapping of two numbers

>>> a, b = 10, 20
>>> print(a, b)
10 20

>>> a, b = b, a
>>> print(a, b)
20 10

 


Reversing a String in Python

Python | Reversing a String

>>> x = 'PythonWorld'
>>> print(x[: : -1])
dlroWnohtyP

 


Python join all items of a list to convert into a single string

Python | Join all items of a list to convert into a single string

>>> x = ["Python", "Online", "Training"]
>>> print(" ".join(x))
Python Online Training

 


python return multiple values from functions

Python | Return multiple values from functions

>>> def A():
	return 2, 3, 4

>>> a, b, c = A()

>>> print(a, b, c)
2 3 4

 


Python Print String N times

Python | Print String N times

>>> s = 'Python'
>>> n = 5

>>> print(s * n)
PythonPythonPythonPythonPython

 


Python check the memory usage of an object

Python | Check the memory usage of  an object

>>> import sys
>>> x = 100

>>> print(sys.getsizeof(x))
28