Google colab file download failed to fetch error

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I encountered the same problem (MessageError: TypeError: Failed to fetch) while using colab.

then, I split file operations into different code units in a colab notebook; I put file open, write, close in one code unit, and use files.download() in the subsequent code unit.

the problem is gone!

The problem is that the file is not finished being written by the time google attempts to "fetch" the file.
Simple solution:

with open('sampleDictionary.json', 'w') as f:
  json.dump(dict, f)
time.sleep(10)
files.download('sampleDictionary.json')

More complicated solution could be put a for loop with a try catch statement for files.download, and then put a sleep in the catch. Keep a max loop time in case the file is never completed.