Fix Python – Remove an item from a dictionary when its key is unknown

Question 1

What is the best way to remove an item from a dictionary by value, i.e. when the item’s key is unknown? Here’s a simple approach:

for key, item in some_dict.items():
    if item is item_to_remove:
        del some_dict[key]

Are there better ways? Is there anything wrong with mutating (deleting items) from the dictionary while iterating it?

Question 2

Be aware that you’re currently testing for object identity (is only returns True if both operands are represented by the same object in memory – this is not always the case with two object that compare equal with ==). If you are doing this on purpose, then you could rewrite your code as

some_dict = {key: value for key, value in some_dict.items() 
             if value is not value_to_remove}

But this may not do what you want:

>>> some_dict = {1: "Hello", 2: "Goodbye", 3: "You say yes", 4: "I say no"}
>>> value_to_remove = "You say yes"
>>> some_dict = {key: value for key, value in some_dict.items() if value is not value_to_remove}
>>> some_dict
{1: 'Hello', 2: 'Goodbye', 3: 'You say yes', 4: 'I say no'}
>>> some_dict = {key: value for key, value in some_dict.items() if value != value_to_remove}
>>> some_dict
{1: 'Hello', 2: 'Goodbye', 4: 'I say no'}

So you probably want != instead of is not.

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