Justin Joyce

Practical tips and tutorials about software development.

Python for loops

For loops in Python are one of many features that make Python so popular; they’re as close to plain English as you can get when writing software. They generally look like this:

for element in iterable:
	# do things with element

What is an iterable? According to the Python docs an iterable is:

An object capable of returning its members one at a time. Examples of iterables include all sequence types (such as list, str, and tuple) and some non-sequence types like dict, file objects, …

In other words, an “iterable” is a “thing that can be looped through”.

Let’s do some examples.

Python “for in” loop

This very English-sounding for loop construction is one of the reasons I love Python. It will perform a single iteration for each element in your iterable:

# lists
for num in [1, 2, 3]:
	print(num)
# 1
# 2
# 3

Most iterable types work exactly how you’d expect:

# sets
for num in set([1, 2, 3]):
	print(num)
# 1
# 2
# 3

Even strings work the same:

# strings
for letter in "abc":
	print(letter)
# a
# b
# c

The continue statement

Similar to many other languages, continue will skip to the next iteration of a loop:

for num in [1, 2, 3, 4]:
	if num == 3:
    	continue
    print(num)
# 1
# 2
# 4

The break statement

The break statement does what its name implies, breaks the loop:

for num in [1, 2, 3, 4]:
	if num == 3:
    	break
    print(num)
# 1
# 2

If you have nested for loops, break will only break the innermost loop:

def nested():
	for num in [1, 2, 3]:
		for char in "abc":
    		if char == "b":
        		break
	        print(num, char)

# We won't see 'b' or 'c'
# since we 'break' before printing 'b'
nested()
# 1 a
# 2 a
# 3 a

The else statement

In a for loop, else will execute code at the end of the loop:

def loop_with_else():
	for num in [1, 2, 3]:
    	print(num)
    else:
        print("else block")
    
    print("done")
        
loop_with_else()
# 1
# 2
# 3
# "else block"
# "done"

However, the else block will not execute if you have a break or return statement inside your loop:

def loop_with_else():
    for num in [1, 2, 3]:
        if num == 2:
            break
        print(num)
    else:
        print("else block")

    print("done")

loop_with_else()
# 1
# "done"

Return from within for loops

Another way to break out of for loops is via the return keyword. Unlike break, return will break out of all loops contained within its parent function:

def nested():
    for num in [1, 2, 3]:
        for char in "abc":
            if char == "b":
                return "returned"
            print(num, char)

# We won't see numbers 2 or 3
# Since we return at loop 1,b
nested()
# 1 a
"returned"

Python for loop with dicts

Looping through dicts might require a small tweak. By default, using for in with a dictionary will only get you the keys:

my_dict = {"first_name": "justin", "last_name": "joyce"}

for thing in my_dict:
	print(thing)
# first_name
# last_name

However, this is an often-used pattern with dictionaries, and with one small addition we can access both keys and values—we need dict.items():

my_dict = {"first_name": "justin", "last_name": "joyce"}

for k, v in my_dict.items():
	print(f"Key is: {k}, Value is: {v}")
# Key is: first_name, Value is: justin
# Key is: last_name, Value is: joyce

This is exactly how you can invert a Python object, which I cover in another post.

Looping with the range function

You’ll sometimes see examples using python’s range() builtin function:

for x in range(3):
	print(x)
# 0
# 1
# 2

Using range is less common than the for in structure in my experience, but you’ll see both.

Unpacking nested iterables

Unpacking nested iterables within a single statement like this might not be the best idea, but Python will allow it:

def unpacking():
    for num, char in [(1, "a"), (2, "b"), (3, "c")]:
        print(num, char)

unpacking()
# 1 a
# 2 b
# 3 c

If you find yourself doing this you might want to consider refactoring—this kind of thing gets confusing quickly.

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