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Adam Huguet
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Asked: 2022-05-12T09:09:14+00:00 In:

Quick handling of the runtimeerror: dictionary changed size during iteration error

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For the “runtimeerror: dictionary changed size during iteration” problem. I tried to fix it, but It doesn’t work and returns the result I want. Here is my program:

d = {'a': [1], 'b': [1, 2], 'c': [], 'd':[]}
for i in d:
  if not d[i]:
  d.pop(i)

and

RuntimeError: dictionary changed size during iteration

has occurred. I’ve checked the entire command line but still can’t find the mistake.

runtimeerror
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2 Answers

  1. Best Answer
    Expert

    The cause: The reason for this error is that while a dictionary changing during for loop, you can not  iterate through it.

    Solution: Let’s make a casting to list and iterate over that list as below:

    for key in list(d): 
         if not d[key]: 
                d.pop(key)
    • 24

  2. Only copy is required

    You can iterate over the dictionary fields and change the desired d on the fly. It works with every Python version so it’s easier to understand. 

    In [1]: d = {'a': [1], 'b': [1, 2], 'c': [], 'd':[]}

In [2]: for i in d.copy():
 ...: if not d[i]:
 ...: d.pop(i)
 ...: 

In [3]: d
Out[3]: {'a': [1], 'b': [1, 2]}

    (BTW) Generally, to iterate over the copy of your data structures, instead of .copy for dictionary or slicing [:] for lists you can use import copy->copy.copy (for shallow copies which are equivalent to copy that’s supported by dictionaries, or slicing [:] that’s supported by lists) on your data structure.

    • 12

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