BUG: Always return a real dtype from linalg.cond (gh-18304) #29333
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Addresses numpygh-18304. The condition number of a matrix is the product of two norms, which are always non-negative and real-valued, so the condition number itself should be non-negative and real-valued. This commit returns the proper real dtype from `linalg.cond`, and includes tests for the condition number of a complex matrix in various norms.
seberg
reviewed
Jul 7, 2025
This commit addresses a reviewer comment on the blanket application of `abs(r)`. It specifically ensures the return type of complex-valued matrices will be the corresponding real type.
seberg
approved these changes
Jul 8, 2025
seberg
reviewed
Jul 8, 2025
| @@ -2011,6 +2011,7 @@ def cond(x, p=None): | |||
| # contain nans in the entries where inversion failed. | |||
| _assert_stacked_square(x) | |||
| t, result_t = _commonType(x) | |||
| result_t = _realType(result_t) # condition number is always real | |||
| signature = 'D->D' if isComplexType(t) else 'd->d' | |||
| with errstate(all='ignore'): | |||
| invx = _umath_linalg.inv(x, signature=signature) | |||
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So, I think this is fine. Your other comment (deleted now) is not wrong, in that it would be fine to cast invx to the original result_t to achieve the right thing.
(Norm might in some paths end up casting back and forth then, but it would work too.)
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I agree that a release note is probably not needed. There is functionally no difference, since the imaginary part was always 0 anyway. There are no member functions that differ between |
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Hi, just checking in on the status of this PR. Let me know if you need me to make any more changes before merging. |
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No, nothing else, just forgot. I think this is a fine solution, do you feel much about back-porting it? Doesn't seem like a bad behavior change, so I guess we can probably just do it, but the bug was probably also around forever anyway. |
charris
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Aug 2, 2025
…py#29333) * BUG: Always return a real dtype from linalg.cond. Addresses numpygh-18304. The condition number of a matrix is the product of two norms, which are always non-negative and real-valued, so the condition number itself should be non-negative and real-valued. This commit returns the proper real dtype from `linalg.cond`, and includes tests for the condition number of a complex matrix in various norms. * ENH: Change type of complex results only. This commit addresses a reviewer comment on the blanket application of `abs(r)`. It specifically ensures the return type of complex-valued matrices will be the corresponding real type.
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Thanks. I will backport it to the v2.x versions since v1.x is reaching end-of-life in about a month anyway. |
broesler
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Aug 6, 2025
…py#29333) * BUG: Always return a real dtype from linalg.cond. Addresses numpygh-18304. The condition number of a matrix is the product of two norms, which are always non-negative and real-valued, so the condition number itself should be non-negative and real-valued. This commit returns the proper real dtype from `linalg.cond`, and includes tests for the condition number of a complex matrix in various norms. * ENH: Change type of complex results only. This commit addresses a reviewer comment on the blanket application of `abs(r)`. It specifically ensures the return type of complex-valued matrices will be the corresponding real type.
broesler
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Aug 6, 2025
…py#29333) * BUG: Always return a real dtype from linalg.cond. Addresses numpygh-18304. The condition number of a matrix is the product of two norms, which are always non-negative and real-valued, so the condition number itself should be non-negative and real-valued. This commit returns the proper real dtype from `linalg.cond`, and includes tests for the condition number of a complex matrix in various norms. * ENH: Change type of complex results only. This commit addresses a reviewer comment on the blanket application of `abs(r)`. It specifically ensures the return type of complex-valued matrices will be the corresponding real type.
broesler
added a commit
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Aug 6, 2025
…py#29333) * BUG: Always return a real dtype from linalg.cond. Addresses numpygh-18304. The condition number of a matrix is the product of two norms, which are always non-negative and real-valued, so the condition number itself should be non-negative and real-valued. This commit returns the proper real dtype from `linalg.cond`, and includes tests for the condition number of a complex matrix in various norms. * ENH: Change type of complex results only. This commit addresses a reviewer comment on the blanket application of `abs(r)`. It specifically ensures the return type of complex-valued matrices will be the corresponding real type.
Closed
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We only backport from main to the current release. "Support" means we post wheels on pip, possibly built with newer libraries or compiler versions, and check that they still pass the tests. |
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Addresses gh-18304.
The condition number of a matrix is the product of two norms, which are
always non-negative and real-valued, so the condition number itself
should be non-negative and real-valued. This commit returns the proper
real dtype from
linalg.cond, and includes tests for the conditionnumber of a complex matrix in various norms.
The existing docstring for
linalg.condalready states that the return value iseither a float or inf, so the function behavior is now consistent with the
docstring.