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

Understanding List Removal During Iteration in Python

While iterating over a list, it may be necessary to remove elements. However, doing so can lead to unexpected behaviors if the removal is not handled correctly.

Consider the Python code:

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

This code produces the output [0, 2, 3, 5, 6, 8, 9], instead of the expected [0, 2, 4, 6, 8]. This anomaly arises because of the way Python handles mutable objects while iterating.

Visual Explanation of the Issue

The sequence of operations can be visualized as follows:

[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
                     ^            

No Error Indication

Python does not issue an error when the underlying iterator is modified because determining when and how a mutation would affect the iterator would require extensive runtime analysis, significantly slowing down the execution.

Behavior Consistency

The behavior described in this example has remained consistent across versions of Python, ensuring predictable execution of code that modifies mutable objects during iteration.

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