Dictionary infinite loop exiting unexpectedly
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:
- I loop through the key's value in the dictionary
- I delete that entry.
- The current counter position in the loop
+ 1
will be the new key with valueNone
which updates the dictionary. - 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?
python dictionary infinite-loop
add a comment |
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:
- I loop through the key's value in the dictionary
- I delete that entry.
- The current counter position in the loop
+ 1
will be the new key with valueNone
which updates the dictionary. - 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?
python dictionary infinite-loop
4
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the4
went into a hash bucket that had already been iterated over.
– jasonharper
1 hour ago
1
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the5
to the left of the iterator position.
– user2357112
50 mins ago
2
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago
add a comment |
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:
- I loop through the key's value in the dictionary
- I delete that entry.
- The current counter position in the loop
+ 1
will be the new key with valueNone
which updates the dictionary. - 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?
python dictionary infinite-loop
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:
- I loop through the key's value in the dictionary
- I delete that entry.
- The current counter position in the loop
+ 1
will be the new key with valueNone
which updates the dictionary. - 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?
python dictionary infinite-loop
python dictionary infinite-loop
edited 25 mins ago
benvc
4,6571321
4,6571321
asked 1 hour ago
Suraj KothariSuraj Kothari
686416
686416
4
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the4
went into a hash bucket that had already been iterated over.
– jasonharper
1 hour ago
1
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the5
to the left of the iterator position.
– user2357112
50 mins ago
2
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago
add a comment |
4
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the4
went into a hash bucket that had already been iterated over.
– jasonharper
1 hour ago
1
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the5
to the left of the iterator position.
– user2357112
50 mins ago
2
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago
4
4
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the
4
went into a hash bucket that had already been iterated over.– jasonharper
1 hour ago
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the
4
went into a hash bucket that had already been iterated over.– jasonharper
1 hour ago
1
1
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the
5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the 5
to the left of the iterator position.– user2357112
50 mins ago
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the
5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the 5
to the left of the iterator position.– user2357112
50 mins ago
2
2
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago
add a comment |
2 Answers
2
active
oldest
votes
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
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
add a comment |
I just tested your code in python2 and python3
python3 output
0,1,2,3,4
python2
0,1,2,3,4,5,6,7
One thing comes to mind that could be going on. Either there is only a certain amount of memory being allocated in your dictionary when you create the first key value and when you delete the key value we do not allocate any memory or deallocate the memory you are just removing the value. Once all the allocated memory is used it exits. Because if you run without that del you will get this error
RuntimeError: dictionary changed size during iteration
So python creates enough memory for that key value and a few more and once it is used up theres no more memory allocated for your dictionary.
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
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
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
add a comment |
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
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
add a comment |
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
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
edited 24 mins ago
answered 1 hour ago
benvcbenvc
4,6571321
4,6571321
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
add a comment |
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
Thanks for the tip on dictionaries, but the code below was sort of unnecessary. I didn't really want an infinite loop in my code, just a fun project of coming up with odd ways, but turns out mines didn't work. Thanks anyways.
– Suraj Kothari
40 mins ago
1
1
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
@SurajKothari - added the example in case someone else ends up looking at your question in an effort to generate an "enumerated" infinite loop (or more usefully an indefinite loop where the breakpoint is determined by some other operation at runtime) similar to your initial attempt.
– benvc
35 mins ago
add a comment |
I just tested your code in python2 and python3
python3 output
0,1,2,3,4
python2
0,1,2,3,4,5,6,7
One thing comes to mind that could be going on. Either there is only a certain amount of memory being allocated in your dictionary when you create the first key value and when you delete the key value we do not allocate any memory or deallocate the memory you are just removing the value. Once all the allocated memory is used it exits. Because if you run without that del you will get this error
RuntimeError: dictionary changed size during iteration
So python creates enough memory for that key value and a few more and once it is used up theres no more memory allocated for your dictionary.
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
add a comment |
I just tested your code in python2 and python3
python3 output
0,1,2,3,4
python2
0,1,2,3,4,5,6,7
One thing comes to mind that could be going on. Either there is only a certain amount of memory being allocated in your dictionary when you create the first key value and when you delete the key value we do not allocate any memory or deallocate the memory you are just removing the value. Once all the allocated memory is used it exits. Because if you run without that del you will get this error
RuntimeError: dictionary changed size during iteration
So python creates enough memory for that key value and a few more and once it is used up theres no more memory allocated for your dictionary.
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
add a comment |
I just tested your code in python2 and python3
python3 output
0,1,2,3,4
python2
0,1,2,3,4,5,6,7
One thing comes to mind that could be going on. Either there is only a certain amount of memory being allocated in your dictionary when you create the first key value and when you delete the key value we do not allocate any memory or deallocate the memory you are just removing the value. Once all the allocated memory is used it exits. Because if you run without that del you will get this error
RuntimeError: dictionary changed size during iteration
So python creates enough memory for that key value and a few more and once it is used up theres no more memory allocated for your dictionary.
I just tested your code in python2 and python3
python3 output
0,1,2,3,4
python2
0,1,2,3,4,5,6,7
One thing comes to mind that could be going on. Either there is only a certain amount of memory being allocated in your dictionary when you create the first key value and when you delete the key value we do not allocate any memory or deallocate the memory you are just removing the value. Once all the allocated memory is used it exits. Because if you run without that del you will get this error
RuntimeError: dictionary changed size during iteration
So python creates enough memory for that key value and a few more and once it is used up theres no more memory allocated for your dictionary.
edited 1 hour ago
answered 1 hour ago
turtlesalldayturtlesallday
271112
271112
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
add a comment |
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
Are you sure Python 3 gives 8 iterations? Mines gave 5 (0 to 4) in Python 3.6.1
– Suraj Kothari
1 hour ago
1
1
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
I just tested it and perhaps you have the versions the other way around. Python 2.7 gives (0 to 7) and Python 3 gives (0 to 4)
– Suraj Kothari
1 hour ago
add a comment |
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4
Mutating an object that you're iterating over is pretty much guaranteed to give you unpredictable results. In your specific case, I would guess that the
4
went into a hash bucket that had already been iterated over.– jasonharper
1 hour ago
1
@jasonharper: CPython 3.6 is when they introduced the new compact dict implementation, so bucket locations wouldn't have this effect on 3.6. This looks like the dict needed a rebuild after the
5
was inserted, which would have re-compacted the compact array of entries and shifted the entry for the5
to the left of the iterator position.– user2357112
50 mins ago
2
You are making unwarranted assumptions about the dictionary iterator. The only assumption that you should make is that it isn't guaranteed to work if you add/remove from the dictionary while iterating over it. What actually ends up happening will depend on various implementation details.
– juanpa.arrivillaga
43 mins ago