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A lens library for python

Project description

Lenses is a python library that helps you to manipulate large data-structures without mutating them. It is inspired by the lenses in Haskell, although it’s much less principled and the api is more suitable for python.


You can install the latest version from pypi using pip like so:

pip install lenses

You can uninstall similarly:

pip uninstall lenses


The lenses library makes liberal use of docstrings, which you can access as normal with the pydoc shell command, the help function in the repl, or by reading the source yourself.

Most users will only need the docs from lenses.UnboundLens. If you want to add hooks to allow parts of the library to work with custom objects then you should check out the lenses.hooks module. Most of the fancy lens code is in the lenses.optics module for those who are curious how everything works.

Some examples are given in the examples folder and the documentation is available on ReadTheDocs.


>>> from pprint import pprint
>>> from lenses import lens
>>> data = [{'name': 'Jane', 'scores': ['a', 'a', 'b', 'a']},
...         {'name': 'Richard', 'scores': ['c', None, 'd', 'c']},
...         {'name': 'Zoe', 'scores': ['f', 'f', None, 'f']}]
>>> format_scores = lens.Each()['scores'].Each().Instance(str).call_upper()
>>> cheat = lens[2]['scores'].Each().set('a')
>>> corrected = format_scores(data)
>>> pprint(corrected)
[{'name': 'Jane', 'scores': ['A', 'A', 'B', 'A']},
 {'name': 'Richard', 'scores': ['C', None, 'D', 'C']},
 {'name': 'Zoe', 'scores': ['F', 'F', None, 'F']}]
>>> cheated = format_scores(cheat(data))
>>> pprint(cheated)
[{'name': 'Jane', 'scores': ['A', 'A', 'B', 'A']},
 {'name': 'Richard', 'scores': ['C', None, 'D', 'C']},
 {'name': 'Zoe', 'scores': ['A', 'A', 'A', 'A']}]

The definition of format_scores means “for each item in the data take the value with the key of 'scores' and then for each item in that list that is an instance of str, call its upper method on it”. That one line is the equivalent of this code:

def format_scores(data):
    results = []
    for entry in data:
        result = {}
        for key, value in entry.items():
            if key == 'scores':
                new_value = []
                for letter in value:
                    if isinstance(letter, str):
                result[key] = new_value
                result[key] = value
    return results

Now, this code can be simplified using comprehensions. But comprehensions only work with lists, dictionaries, and sets, whereas the lenses library can work with arbitrary python objects.

Here’s an example that shows off the full power of this library:

>>> from lenses import lens
>>> state = (("foo", "bar"), "!", 2, ())
>>> lens.Recur(str).Each().Filter(lambda c: c <= 'm').Parts().call_mut_reverse()(state)
(('!oo', 'abr'), 'f', 2, ())

This is an example from the Putting Lenses to Work talk about the haskell lenses library by John Wiegley. We extract all the strings inside of state, extract the characters, filter out any characters that come after 'm' in the alphabet, treat these characters as if they were a list, reverse that list, before finally placing these characters back into the state in their new positions.

This example is obviously very contrived, but I can’t even begin to imagine how you would do this in python code without lenses.


python-lenses is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see

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