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# itertools.groupby() in Python

• Difficulty Level : Medium
• Last Updated : 30 Jan, 2020

Prerequisites: Python Itertools

Python’s Itertool is a module that provides various functions that work on iterators to produce complex iterators. This module works as a fast, memory-efficient tool that is used either by themselves or in combination to form iterator algebra.

#### Itertools.groupby()

This method calculates the keys for each element present in iterable. It returns key and iterable of grouped items.

Syntax: itertools.groupby(iterable, key_func)

Parameters:
iterable: Iterable can be of any kind (list, tuple, dictionary).
key: A function that calculates keys for each element present in iterable.

Return type: It returns consecutive keys and groups from the iterable. If the key function is not specified or is None, key defaults to an identity function and returns the element unchanged.

Example 1:

 `# Python code to demonstrate ``# itertools.groupby() method `` ` ` ` `import` `itertools`` ` ` ` `L ``=` `[(``"a"``, ``1``), (``"a"``, ``2``), (``"b"``, ``3``), (``"b"``, ``4``)]`` ` `# Key function``key_func ``=` `lambda` `x: x[``0``]`` ` `for` `key, group ``in` `itertools.groupby(L, key_func):``    ``print``(key ``+` `" :"``, ``list``(group))`

Output:

```a : [('a', 1), ('a', 2)]
b : [('b', 3), ('b', 4)]
```

Example 2 :

 `# Python code to demonstrate ``# itertools.groupby() method `` ` ` ` `import` `itertools`` ` ` ` `a_list ``=` `[(``"Animal"``, ``"cat"``), ``          ``(``"Animal"``, ``"dog"``), ``          ``(``"Bird"``, ``"peacock"``), ``          ``(``"Bird"``, ``"pigeon"``)]`` ` ` ` `an_iterator ``=` `itertools.groupby(a_list, ``lambda` `x : x[``0``])`` ` `for` `key, group ``in` `an_iterator:``    ``key_and_group ``=` `{key : ``list``(group)}``    ``print``(key_and_group)`

Output

```{'Animal': [('Animal', 'cat'), ('Animal', 'dog')]}
{'Bird': [('Bird', 'peacock'), ('Bird', 'pigeon')]}
```

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