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I was running a piece of code that unexpectedly gave a logic error at one part of the program. When investigating the section, I created a test file to test the set of statements being run and found out an unusual bug that seems very odd.

I tested this simple code:

array = [1, 2, 2, 4, 5] # Original array

f = (x for x in array if array.count(x) == 2) # Filters original

array = [5, 6, 1, 2, 9] # Updates original to something else



print(list(f)) # Outputs filtered

And the output was:

>>> 

Yes, nothing. I was expecting the filter comprehension to get items in the array with a count of 2 and output this, but I didn't get that:

# Expected output

>>> [2, 2]

When I commented out the third line to test it once again:

array = [1, 2, 2, 4, 5] # Original array

f = (x for x in array if array.count(x) == 2) # Filters original

### array = [5, 6, 1, 2, 9] # Ignore line



print(list(f)) # Outputs filtered

The output was correct (you can test it for yourself):

>>> [2, 2]

At one point I outputted the type of the variable 'f':

array = [1, 2, 2, 4, 5] # Original array

f = (x for x in array if array.count(x) == 2) # Filters original

array = [5, 6, 1, 2, 9] # Updates original



print(type(f))

print(list(f)) # Outputs filtered

And I got:

>>> <class 'generator'>

>>> 

TL;DR: Why is updating a list in Python changing the output of another generator variable? This seems very odd to me.

As others have mentioned Python generators are lazy. When this line is run:

f = (x for x in array if array.count(x) == 2) # Filters original

nothing actually happens yet. You've just declared how the generator function f will work. Array is not looked at yet. Then, you create a new array that replaces the first one, and finally when you call

print(list(f)) # Outputs filtered

the generator now needs the actual values and starts pulling them from the generator f. But at this point, array already refers to the second one, so you get an empty list.

If you need to reassign the list, and can't use a different variable to hold it, consider creating the list instead of a generator in the second line:

f = [x for x in array if array.count(x) == 2] # Filters original

...

print(f)

Generators are lazy, they won't be evaluated until you iterate through them. In this case that's at the point you create the list with the generator as input, at the print.

You are not using a generator correctly if this is the primary use of this code. Use a list comprehension instead of a generator comprehension. Just replace the parentheses with brackets. It evaluates to a list if you don't know.

array = [1, 2, 2, 4, 5]

f = [x for x in array if array.count(x) == 2]

array = [5, 6, 1, 2, 9]



print(f)

#[2, 2]

You are getting this response because of the nature of a generator. You're calling the generator when it't contents will evaluate to

Generator evaluation is "lazy" -- it doesn't get executed until you actualize it with a proper reference. With your line:

Look again at your output with the type of f: that object is a generator, not a sequence. It's waiting to be used, an iterator of sorts.

Your generator isn't evaluated until you start requiring values from it. At that point, it uses the available values at that point, not the point at which it was defined.

Code to "make it work"

That depends on what you mean by "make it work". If you want f to be a filtered list, then use a list, not a generator:

f = [x for x in array if array.count(x) == 2] # Filters original

Generators are lazy and your newly defined array is used when you exhaust your generator after redefining. Therefore, the output is correct. A quick fix is to use a list comprehension by replacing parentheses () by brackets .

Moving on to how better to write your logic, counting a value in a loop has quadratic complexity. For an algorithm that works in linear time, you can use collections.Counter to count values, and keep a copy of your original list:

from collections import Counter



array = [1, 2, 2, 4, 5]   # original array

counts = Counter(array)   # count each value in array

old_array = array.copy()  # make copy

array = [5, 6, 1, 2, 9]   # updates array



# order relevant

res = [x for x in old_array if counts[x] >= 2]

print(res)

# [2, 2]



# order irrelevant

from itertools import chain

res = list(chain.from_iterable([x]*count for x, count in counts.items() if count >= 2))

print(res)

# [2, 2]

Notice the second version doesn't even require old_array and is useful if there is no need to maintain ordering of values in your original array.