Fix coefficient method for modular forms

[DavidAyotte]

Currently, the coefficient method appear in the TAB-completion for a modular form, but an error occur when one tries to use it.

sage: f = ModularForms(1, 12).0
sage: f.coefficient # hit TAB
                    f.coefficient
                    f.coefficients
sage: f.coefficient(10)
Traceback (most recent call last):
...
AttributeError: 'Sequence_generic' object has no attribute 'keys'

Component: modular forms

Keywords: modular forms coefficient

Author: David Ayotte

Branch/Commit: 0071130

Reviewer: Vincent Delecroix

Issue created by migration from https://trac.sagemath.org/ticket/32553

[DavidAyotte]

Branch: u/gh-DavidAyotte/fix_coefficient_method_for_modular_forms

[DavidAyotte]

New commits:

a4242a6

added coefficient method in sage/modular/modform/element.py

[DavidAyotte]

Author: David Ayotte

[DavidAyotte]

Commit: a4242a6

[mkoeppe]

comment:3

Stalled in needs_review or needs_info; likely won't make it into Sage 9.5.

[videlec]

Reviewer: Vincent Delecroix

[videlec]

comment:4

The conversion int(n) is not ideal since n=3/2 would be converted to 1.

[DavidAyotte]

comment:5

Hello Vincent, thank you very much for reviewing this!

I've tried the code with your example and, strangely, I got:

sage: f = ModularForms(1, 12).0
sage: f.coefficient(3/2)
Traceback (most recent call last):
...
TypeError: no conversion of this rational to integer

I have no idea why this happens, because you are indeed right, in the command line interface we do have:

sage: int(3/2)
1

[sagetrac-git]

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

6797e1d

fix integer type conversion

[sagetrac-git]

Changed commit from a4242a6 to 6797e1d

[videlec]

comment:8

The error comes from

sage: f.q_expansion(3/2)
Traceback (most recent call last):
...
TypeError: no conversion of this rational to integer

[videlec]

comment:9

I would rather do

n = Integer(n)
return self.q_expansion(n+1)[n]

so that the error message will be clearer, ie pointing to the line n = ZZ(n).

[sagetrac-git]

Changed commit from 6797e1d to 0071130

[sagetrac-git]

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

0071130

minor changes for clearer error message

[DavidAyotte]

comment:11

Replying to @videlec:

I would rather do

n = Integer(n)
return self.q_expansion(n+1)[n]

so that the error message will be clearer, ie pointing to the line n = ZZ(n).

Thank you for the suggestion! Happy new year!

[videlec]

comment:12

Happy new year!

[DavidAyotte]

comment:13

I might be wrong, but the failed doctest don't seem related to this ticket, see #32782 and #33026.

[videlec]

comment:14

Sorry. I wanted to set it to positive review the first time. Wrong manipulation.

[vbraun]

Changed branch from u/gh-DavidAyotte/fix_coefficient_method_for_modular_forms to 0071130