Dictionary infinite loop is exiting unexpectedly

Question

I was experimenting with various ways of creating an infinite loop in Python (other than the usual while True), and came up with this idea:

x = {0: None}  for i in x:     del x[i]     x[i+1] = None  # Value doesn't matter, so I set it to None     print(i) 

On paper, I traced out the way this would infinitely loop:

1. I loop through the key's value in the dictionary
2. I delete that entry.
3. The current counter position in the loop + 1 will be the new key with value None which updates the dictionary.
4. I output the current counter.

This, in my head, should output the natural numbers in a sort of infinite loop fashion:

0 1 2 3 4 5 . . . 

I thought this idea was clever, however when I run it on Python 3.6, it outputs:

0 1 2 3 4 

Yes, it somehow stopped after 5 iterations. Clearly, there is no base condition or sentinel value in the code block of the loop, so why is Python only running this code 5 times?

Answer 1

There is no guarantee that you will iterate over all your dict entries if you mutate it in your loop. From the docs:

Iterating views while adding or deleting entries in the dictionary may raise a RuntimeError or fail to iterate over all entries.

You could create an "enumerated" infinite loop similar to your initial attempt using itertools.count(). For example:

from itertools import count  for i in count():     print(i)     # don't run this without some mechanism to break the loop, i.e.     # if i == 10:     #     break  # OUTPUT # 0 # 1 # 2 # ...and so on 

Answer 2

In this case, like @benvc wrote, this is not guaranteed to work. But in case you wonder why does it work in C-Python:

The C-Python implementation destroys the dict object after some inserts and copies it to a new space in memory. It does not care about deletions. So when this happens, the loop notices it and breaking with an exception.

Check out this link if you want to read more about this, and many other interesting python internals here.

https://github.com/satwikkansal/wtfpython#-modifying-a-dictionary-while-iterating-over-it