How to understand a one-line code that extract subset dictionary with selected key and value pairs
Question:
I am working on a Python project. I am creating a subset dictionary from the main dictionary, the subset dictionary only takes the key, value pairs that are under keys: ‘E’, ‘O’, ‘L’.
I found this code is working perfectly:
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() & {'E', 'O', 'L'}}
However, I would like to understand how it works. How could this be expressed in multiple lines of code? I guess it is something like: for key in ….
Answers:
In short python has comprehensions that can provide one liner for datatype construction using for loops.
Here, this is an example of dictionary comprehension.
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() & {'E', 'O', 'L'}}
This code can be extended to:
output = dict()
for key in self._the_main_dict.keys() & {'E', 'O', 'L'}:
output[key] = self._the_main_dict[key]
output is the response provided by above dictionary comprehension.
self._the_main_dict.keys() & {'E', 'O', 'L'}. If you’re confused on this. This is just an Intersection operation between two sets. Yeah! Sets and Venn diagram you used to study in mathematics 😀
This works by creating an intersection of the dict’s keys with the set containing E, O and L. In long form, this could be written as:
all_keys = self._the_main_dict.keys()
keys_of_interest = {'E', 'O', 'L'}
filtered_keys = all_keys & keys_of_interest
# alternatively:
filtered_keys = keys_of_interest.intersection(all_keys)
These keys are then iterated over to create the resulting dict via a dictionary comprehension. A clearer way would be to explicitly specify the condition rather than implicitly through the merging step:
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() if key in {'E', 'O', 'L'}}
The keys() method returns a set-like object containing the keys of the dictionary. Sets use the & operator to perform intersection, so
self._the_main_dict.keys() & {'E', 'O', 'L'}
is equivalent to
self._the_main_dict.keys().intersection({'E', 'O', 'L'})
and returns the subset of the keys that are in the {'E', 'O', 'L'} set.
Then the rest of the dictionary comprehension creates a new dictionary containing key: self._the_main_dict[key] for those keys.
A more common way to write this would be:
{key: self._the_main_dict[key]
for key in self._the_main_dict.keys()
if key in {'E', 'O', 'L'}}
I am working on a Python project. I am creating a subset dictionary from the main dictionary, the subset dictionary only takes the key, value pairs that are under keys: ‘E’, ‘O’, ‘L’.
I found this code is working perfectly:
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() & {'E', 'O', 'L'}}
However, I would like to understand how it works. How could this be expressed in multiple lines of code? I guess it is something like: for key in ….
In short python has comprehensions that can provide one liner for datatype construction using for loops.
Here, this is an example of dictionary comprehension.
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() & {'E', 'O', 'L'}}
This code can be extended to:
output = dict()
for key in self._the_main_dict.keys() & {'E', 'O', 'L'}:
output[key] = self._the_main_dict[key]
output is the response provided by above dictionary comprehension.
self._the_main_dict.keys() & {'E', 'O', 'L'}. If you’re confused on this. This is just an Intersection operation between two sets. Yeah! Sets and Venn diagram you used to study in mathematics 😀
This works by creating an intersection of the dict’s keys with the set containing E, O and L. In long form, this could be written as:
all_keys = self._the_main_dict.keys()
keys_of_interest = {'E', 'O', 'L'}
filtered_keys = all_keys & keys_of_interest
# alternatively:
filtered_keys = keys_of_interest.intersection(all_keys)
These keys are then iterated over to create the resulting dict via a dictionary comprehension. A clearer way would be to explicitly specify the condition rather than implicitly through the merging step:
{key: self._the_main_dict[key] for key in self._the_main_dict.keys() if key in {'E', 'O', 'L'}}
The keys() method returns a set-like object containing the keys of the dictionary. Sets use the & operator to perform intersection, so
self._the_main_dict.keys() & {'E', 'O', 'L'}
is equivalent to
self._the_main_dict.keys().intersection({'E', 'O', 'L'})
and returns the subset of the keys that are in the {'E', 'O', 'L'} set.
Then the rest of the dictionary comprehension creates a new dictionary containing key: self._the_main_dict[key] for those keys.
A more common way to write this would be:
{key: self._the_main_dict[key]
for key in self._the_main_dict.keys()
if key in {'E', 'O', 'L'}}