Bug in initialization of vector field with rational components

[egourgoulhon]

Initializing the components of a vector field with a rational number as first component leads to an error:

sage: M = Manifold(2, 'M')
sage: X.<x,y> = M.chart()
sage: v = M.vector_field(1/2, -1)
...
IndexError: index n (=1) out of range; it must be 0

while providing the components as a list is OK:

sage: v = M.vector_field([1/2, -1])
sage: v.display()
1/2 d/dx - d/dy

This is due to Rational class having a __getitem__() method (for some reason...) and TensorField._init_components (introduced in #27581) testing its input by

    if hasattr(comp0, '__getitem__'):

Replacing the above line by

    if hasattr(comp0, '__len__') and hasattr(comp0, '__getitem__'):

fixes the bug, since Rational has no __len__ method.

[egourgoulhon]

New commits:

​3142781

Fix bug on initializing a vector field with rational components (trac #29639)

[git]

Branch pushed to git repo; I updated commit sha1. New commits:

​62dc948

#29639: correct the fix

[egourgoulhon]

The fix proposed in comment:1, namely use of

    if isinstance(comp0, (list, tuple)):

failed if comp0 is an object created by vector(...), as revealed by a failed doctest in vectorfield.py. If the latest commit (comment:4), this is replaced by

    if hasattr(comp0, '__len__') and hasattr(comp0, '__getitem__'):

This returns True (as it should) for list, tuple and vector and False (as it should) for a Rational.

[tscrim]

This will work. (Note that matrices also fall under this criteria, but I don't know of a (reasonable) field that would. Number fields, p-adics, polynomials, etc. have a __getitem__ but not a __len__.) Generally I would prefer something more explicit, but I am okay with the generality.

[egourgoulhon]

Replying to tscrim:

This will work. (Note that matrices also fall under this criteria,

That's a good point; it's nice that it works for matrices too.

but I don't know of a (reasonable) field that would. Number fields, p-adics, polynomials, etc. have a __getitem__ but not a __len__.) Generally I would prefer something more explicit,

Me too, but I could not think of something better in that case...

but I am okay with the generality.

Thank you for the review!