How Can I Select Variables in Python Using a String Input?

Selecting Variables by String Name

To select a variable based on a string input, there are several viable approaches.

Dictionary

An ordinary dictionary is often suitable for this task:

get_ext = {'text': ['txt', 'doc'],
           'audio': ['mp3', 'wav'],
           'video': ['mp4', 'mkv']}

get_ext['video']  # returns ['mp4', 'mkv']

Function

If a function is required for specific reasons, you can assign to the get method of the dictionary:

get_ext = get_ext.get  # Equivalent to get_ext = lambda key: get_ext.get(key)
get_ext('video')  # returns ['mp4', 'mkv']

This will return None for unknown keys by default. To raise a KeyError instead, assign to get_ext.__getitem__:

get_ext = get_ext.__getitem__  # Equivalent to get_ext = lambda key: get_ext.__getitem__(key)
get_ext('video')  # returns ['mp4', 'mkv']

Custom Default Value

You can implement a custom default value by wrapping the dictionary in a function:

def get_ext(file_type):
    types = {'text': ['txt', 'doc'],
             'audio': ['mp3', 'wav'],
             'video': ['mp4', 'mkv']}

    return types.get(file_type, [])

Optimizing

To avoid recreating the dictionary on every function call, you can use a class:

class get_ext(object):
    def __init__(self):
        self.types = {'text': ['txt', 'doc'],
                      'audio': ['mp3', 'wav'],
                      'video': ['mp4', 'mkv']}

    def __call__(self, file_type):
        return self.types.get(file_type, [])

get_ext = get_ext()

This allows for easy modification of recognized file types:

get_ext.types['binary'] = ['bin', 'exe']
get_ext('binary')  # returns ['bin', 'exe']

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