The itertools module in the standard library offers a number of tools that for conveniently iterating through iterables.

The groupby() function in the module is used for grouping and sorting data based on a certain key.

Syntax: 
groupby(iterable, key = None)
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iterable Required. An iterable(list, set, tuple, range, etc) containing the elements to be grouped.
key Optional. A function that will be used for grouping the elements of the iterable. 
Parameters:

The groupby() function returns an iterator of tuples in which each tuple contains a key and the corresponding group of elements from the iterable.

ExampleEdit & Run
from itertools import groupby

bodies = [('planet', 'Jupiter'), ("planet", "Earth"), ("planet", "Mars"), ('galaxy', 'Andromeda'), ("galaxy", "Milky way"), ("moon", "Europa"), ("moon", "Lunar")]

for key, group in groupby(bodies, lambda x: x[0]):
    similar_bodies = ", ".join([body[1] for body in group])
    print(key + "(s):  " + similar_bodies + ".")
 Visualize 
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Output:
planet(s):  Jupiter, Earth, Mars.
galaxy(s):  Andromeda, Milky way.
moon(s):  Europa, Lunar.
[Finished in 0.010801333002746105s]

In the above example, bodies is a list of tuples where each tuple contains a celestial body,  first item in each tuple is the body's category and the second is its name.

We used lambda x: x[0] as the key parameter to tell the  groupby() function to use the first item in each tuple as the grouping key.

In the for loop, groupby() returns three (key, group iterator) pairs - once for each unique key. You can use the returned iterator to iterate over each individual item in that group.

Let us see another example:

ExampleEdit & Run

Group people by age

from itertools import groupby

data = [ {'name': 'John', 'age': 25}, {'name': 'Tim', 'age': 20}, {'name': 'Sally', 'age': 30}, {'name': 'Paul', 'age': 25},]

sorted_data = sorted(data, key=lambda x: x['age']) 

grouped_data = groupby(sorted_data, key = lambda x: x['age']) 

for key, group in grouped_data: 
     agemates = ', '.join(person['name'] for person in group)
     print(f'Age {key}: %s'% agemates)
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Output:
Age 20: Tim
Age 25: John, Paul
Age 30: Sally
[Finished in 0.011563248001039028s]

In the above example we have a variable data which is a list of dictionary where each dictionary contains the name and the age of a particular person.

We sorted the dictionaries by age using the builtin sorted() function and then called the groupby() function on the sorted data. Note that the function expects the input iterables to be sorted as per the given key before they are passed for grouping, otherwise,  it may produce inconsistent results.

We then used a for loop to get each key and its group of elements before printing them.