
Python for Loops – Mastering Iteration the Pythonic Way
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Loops are essential in any programming language. Python’s for
loop is simple yet incredibly powerful, allowing you to iterate over lists, strings, dictionaries, sets, and more with elegant syntax.
This guide will cover:
-
What
for
loops are and how they work -
Looping over different data types
-
Using
range()
-
Nested loops
-
Controlling loops with
break
andcontinue
-
Tips and common pitfalls
-
A complete example
✅ What Is a for
Loop?
A for
loop in Python is used to iterate over a sequence (like a list, tuple, dictionary, set, or string).
Syntax
for variable in sequence:
# code block
-
variable
: Temporary placeholder for each item in the sequence. -
sequence
: A collection of items to loop through.
Example 1: Looping Over a List
fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
print(fruit)
Output:
apple
banana
cherry
Example 2: Looping Over a String
for char in "Python":
print(char)
Example 3: Using range()
range()
generates a sequence of numbers, which is often used for looping a specific number of times.
for i in range(5):
print(i)
Output:
0
1
2
3
4
range(5)
produces numbers from 0 to 4 (not inclusive of 5).
range(start, stop, step)
Example
for i in range(1, 10, 2):
print(i)
Output:
1
3
5
7
9
Example 4: Looping Over a Dictionary
student = {"name": "Alice", "age": 20, "grade": "A"}
for key, value in student.items():
print(f"{key}: {value}")
Example 5: Looping Over a Set or Tuple
colors = {"red", "green", "blue"}
for color in colors:
print(color)
Note: Sets are unordered, so output order may vary.
Nested for
Loops
A loop inside another loop.
for i in range(1, 4):
for j in range(1, 4):
print(f"{i} * {j} = {i*j}")
Exiting Loops Early – break
for i in range(5):
if i == 3:
break
print(i)
Output:
0
1
2
Skipping Iterations – continue
for i in range(5):
if i == 2:
continue
print(i)
Output:
0
1
3
4
➕ The else
Clause
Python’s for
loops can have an else
block, which executes only if the loop completes without a break
.
for i in range(3):
print(i)
else:
print("Loop completed successfully!")
Common Pitfalls
Pitfall | Cause | Solution |
---|---|---|
Forgetting : |
SyntaxError | Always use a colon after for |
Modifying the list while looping | Unexpected results | Loop over a copy if modifying |
Assuming order in sets/dicts | Sets/dicts are unordered | Use sorted() if order is needed |
Using = instead of == |
Assignment instead of comparison in if |
Use == for condition checks |
Tips and Best Practices
-
Use
enumerate()
to get index and value:for index, value in enumerate(["a", "b", "c"]): print(index, value)
-
Use
zip()
to loop over multiple lists together:names = ["Alice", "Bob"] scores = [85, 90] for name, score in zip(names, scores): print(f"{name} scored {score}")
-
Use
reversed()
andsorted()
with any iterable:for num in reversed(range(5)): print(num)
Complete Example: Multiplication Table
for i in range(1, 6):
for j in range(1, 6):
print(f"{i * j:2}", end=" ")
print()
Output:
1 2 3 4 5
2 4 6 8 10
3 6 9 12 15
4 8 12 16 20
5 10 15 20 25
What’s Next?
After learning for
loops, you can explore:
-
Comprehensions: List/set/dictionary comprehensions using
for
-
Generators and iterators
-
Looping through files or API results
-
Combining with
if
conditions inside loops
Practice Challenge
Task: Write a program that finds the factorial of a given number using a for
loop.
num = int(input("Enter a number: "))
factorial = 1
for i in range(1, num + 1):
factorial *= i
print("Factorial is", factorial)
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=3Hence, 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