Why Does Removing Elements from a Python List During Iteration Produce Unexpected Results?

Removing from a List While Iterating Over It

When iterating over a list and removing elements, Python's behavior can be puzzling. The code snippet below:

a = list(range(10))
remove = False
for b in a:
    if remove:
        a.remove(b)
    remove = not remove
print(a)

Outputs [0, 2, 3, 5, 6, 8, 9], instead of the expected [0, 2, 4, 6, 8].

Understanding the Iterative Removal Behavior

This output results from the way Python handles mutable sequences during iteration. When remove is called, the underlying list is modified. However, the iterator remains the same and continues traversing the modified list.

The diagram below illustrates the removal process:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]       <-  b = 0; remove? no
 ^
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]       <-  b = 1; remove? yes
    ^
[0, 2, 3, 4, 5, 6, 7, 8, 9]          <-  b = 3; remove? no
       ^
[0, 2, 3, 4, 5, 6, 7, 8, 9]          <-  b = 4; remove? yes
          ^
[0, 2, 3, 5, 6, 7, 8, 9]             <-  b = 6; remove? no
             ^
[0, 2, 3, 5, 6, 7, 8, 9]             <-  b = 7; remove? yes
                ^
[0, 2, 3, 5, 6, 8, 9]                <-  b = 9; remove? no
                   ^

Addressing the Remaining Questions

1. Error Indication: Python does not throw an error because it would incur runtime overhead. It assumes that the programmer knows the consequences of modifying the list during iteration and handles it accordingly.
2. Historical Behavior: This behavior has existed in Python since early versions (at least 1.4), and is consistent with how mutable sequences are typically implemented.

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