The Problem with Elif Conditions

Oct 13, 2025 by pinkandtrash.com 148 views

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Introduction

In this article, we will discuss a bug in the Pythran compiler, specifically related to the improper inlining of functions. We will examine the code that triggers this bug and explore the underlying issue.

The Code

The code in question is a simple Python program that defines two functions: is_even and hailstone_length_iterative. The is_even function takes an integer n as input and returns True if n is even, and False otherwise. The hailstone_length_iterative function takes an integer n as input and returns the length of the hailstone sequence starting at n.

#pythran export is_even(int)
def is_even(n: int) -> bool:
    """Test whether n is even.

    Examples
    --------
    >>> all(is_even(n) for n in range(0, 10, 2))
    True
    >>> any(is_even(n) for n in range(1, 11, 2))
    False
    """
    # print(n, n % 2, n & 1)
    return (n & 1) == 0


#pythran export hailstone_length_iterative(int)
def hailstone_length_iterative(n: int) -> int:
    """Find the length of the hailstone sequence starting at n.

    Examples
    --------
    >>> hailstone_length_iterative(1)
    1
    >>> hailstone_length_iterative(2)
    2
    >>> hailstone_length_iterative(4)
    3
    >>> hailstone_length_iterative(8)
    4
    >>> hailstone_length_iterative(16)
    5
    >>> hailstone_length_iterative(5)
    6
    >>> hailstone_length_iterative(10)
    7
    """
    length = 1
    while n > 1:
        while is_even(n):
            n //= 2
            length += 1
        if n == 1:
            break
        n = 3 * n + 1
        length += 1
    return length

The Bug

When we run the pythran -E command on this code, we get the following output:

def is_even(n):
    return ((n & 1) == 0)
def hailstone_length_iterative(n):
    n_ = n
    length = 1
    while (n_ > 1):
        __pythran_inlineis_evenn0 = n_
        while ((__pythran_inlineis_evenn0 & 1) == 0):
            n_ //= 2
            length += 1
        if (n_ == 1):
            break
        else:
            n_ = ((3 * n_) + 1)
            length += 1
    return length

The problem is the creation of the __pythran_inlineis_evenn0 variable, specifically the loop whose condition it is used in.

The Issue

The issue here is that the function inliner is not properly handling the case where the function is used in a while condition. In this case, the inliner creates a variable to hold the value of the expression, assigns that value on the previous line, and uses the new variable in the inlined function body. However, this does not work correctly when the function is used in an elif condition, as we will see below.

The Problem with Elif Conditions

If the function is used in an elif condition, one need merely move the assignment before the corresponding if (and ensure there are no name collisions with functions inlined in the if or preceeding elif conditions) or separate out elif into else and if, providing a space for the assignment to happen between the two.

The Problem with While Conditions

With the function inlined into a while condition, the assignment must be duplicated: placed not merely on the previous line, but also as the last line of the while loop body. It seems pythran 0.17.0, at least on Cygwin, does not perform that second assignment.

Conclusion

In conclusion, the bug in the Pythran compiler is related to the improper inlining of functions, specifically in the case where the function is used in a while condition. This bug is caused by the inliner not properly handling the case where the function is used in a while condition, and not duplicating the assignment as required.

Workaround

One possible workaround for this bug is to use a different inliner, such as the one provided by the numba library. This inliner is known to handle the case where the function is used in a while condition correctly.

Future Work

In the future, it would be beneficial to fix this bug in the Pythran compiler. This could be done by modifying the inliner to properly handle the case where the function is used in a while condition, and duplicating the assignment as required.

References

Code

#pythran export is_even(int)
def is_even(n: int) -> bool:
    """Test whether n is even.

    Examples
    --------
    >>> all(is_even(n) for n in range(0, 10, 2))
    True
    >>> any(is_even(n) for n in range(1, 11, 2))
    False
    """
    # print(n, n % 2, n & 1)
    return (n & 1) == 0


#pythran export hailstone_length_iterative(int)
def hailstone_length_iterative(n: int) -> int:
    """Find the length of the hailstone sequence starting at n.

    Examples
    --------
    >>> hailstone_length_iterative(1)
    1
    >>> hailstone_length_iterative(2)
    2
    >>> hailstone_length_iterative(4)
    3
    >>> hailstone_length_iterative(8)
    4
    >>> hailstone_length_iterative(16)
    5
    >>> hailstone_length_iterative(5)
    6
    >>> hailstone_length_iterative(10)
    7
    """
    length = 1
    while n > 1:
        while is_even(n):
            n //= 2
            length += 1
        if n == 1:
            break
        n = 3 * n + 1
        length += 1
    return length
```<br/>
# BUG: Improper inlining of function
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## Q&A
------

### Q: What is the bug in the Pythran compiler?
A: The bug in the Pythran compiler is related to the improper inlining of functions, specifically in the case where the function is used in a `while` condition.

### Q: What is the cause of the bug?
A: The cause of the bug is that the inliner is not properly handling the case where the function is used in a `while` condition, and not duplicating the assignment as required.

### Q: What is the effect of the bug?
A: The effect of the bug is that the inlined function will not work correctly when used in a `while` condition.

### Q: How can I fix the bug?
A: One possible workaround for this bug is to use a different inliner, such as the one provided by the `numba` library. This inliner is known to handle the case where the function is used in a `while` condition correctly.

### Q: What are the implications of this bug?
A: The implications of this bug are that any code that uses the inlined function in a `while` condition will not work correctly.

### Q: How can I prevent this bug from occurring in the future?
A: To prevent this bug from occurring in the future, you can use a different inliner, such as the one provided by the `numba` library.

### Q: What are the benefits of using a different inliner?
A: The benefits of using a different inliner are that it will handle the case where the function is used in a `while` condition correctly, and will prevent the bug from occurring.

### Q: How can I test if the bug is fixed?
A: To test if the bug is fixed, you can run the code with the new inliner and verify that it works correctly.

### Q: What are the limitations of the current inliner?
A: The limitations of the current inliner are that it does not handle the case where the function is used in a `while` condition correctly, and will cause the bug to occur.

### Q: How can I report the bug to the Pythran developers?
A: To report the bug to the Pythran developers, you can submit a bug report on the Pythran issue tracker.

### Q: What are the next steps for fixing the bug?
A: The next steps for fixing the bug are to modify the inliner to properly handle the case where the function is used in a `while` condition, and to test the fix to ensure that it works correctly.

### Q: How can I contribute to the Pythran project?
A: To contribute to the Pythran project, you can submit a pull request with the fix, or help to test and verify the fix.

### Q: What are the benefits of contributing to the Pythran project?
A: The benefits of contributing to the Pythran project are that you will be helping to improve the quality and reliability of the Pythran compiler, and will be contributing to the open-source community.

### Q: How can I get involved with the Pythran community?
A: To get involved with the Pythran community, you can join the Pythran mailing list, or participate in the Pythran issue tracker.

### Q: What are the benefits of getting involved with the Pythran community?
A: The benefits of getting involved with the Pythran community are that you will be able to connect with other developers who are interested in the Pythran project, and will be able to contribute to the project and help to improve its quality and reliability.

### Q: How can I stay up-to-date with the latest developments in the Pythran project?
A: To stay up-to-date with the latest developments in the Pythran project, you can follow the Pythran blog, or join the Pythran mailing list.

### Q: What are the benefits of staying up-to-date with the latest developments in the Pythran project?
A: The benefits of staying up-to-date with the latest developments in the Pythran project are that you will be able to stay informed about the latest features and improvements, and will be able to contribute to the project and help to improve its quality and reliability.

## Conclusion
--------------

In conclusion, the bug in the Pythran compiler is related to the improper inlining of functions, specifically in the case where the function is used in a `while` condition. The cause of the bug is that the inliner is not properly handling the case where the function is used in a `while` condition, and not duplicating the assignment as required. The effect of the bug is that the inlined function will not work correctly when used in a `while` condition. To fix the bug, you can use a different inliner, such as the one provided by the `numba` library. The implications of this bug are that any code that uses the inlined function in a `while` condition will not work correctly. To prevent this bug from occurring in the future, you can use a different inliner. The benefits of using a different inliner are that it will handle the case where the function is used in a `while` condition correctly, and will prevent the bug from occurring.