Don't try to call _repr_*_ on a class

[tkf]

fix #816

An MWE

py"""
class Issue816(object):
    def _repr_html_(self):
        return "<h1>Issue816</h1>"
"""

showable("text/html", py"Issue816()")  # => true
showable("text/html", py"Issue816")  # => throws

ERROR: PyError ($(Expr(:escape, :(ccall(#= /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:43 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw))))) <class 'TypeError'>
TypeError("_repr_html_() missing 1 required positional argument: 'self'")

Stacktrace:
 [1] pyerr_check at /home/takafumi/.julia/dev/PyCall/src/exception.jl:62 [inlined]
 [2] pyerr_check at /home/takafumi/.julia/dev/PyCall/src/exception.jl:66 [inlined]
 [3] _handle_error(::String) at /home/takafumi/.julia/dev/PyCall/src/exception.jl:83
 [4] macro expansion at /home/takafumi/.julia/dev/PyCall/src/exception.jl:97 [inlined]
 [5] #110 at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:43 [inlined]
 [6] disable_sigint at ./c.jl:446 [inlined]
 [7] __pycall! at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:42 [inlined]
 [8] _pycall!(::PyObject, ::PyObject, ::Tuple{}, ::Int64, ::Ptr{Nothing}) at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:29
 [9] _pycall! at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:11 [inlined]
 [10] #pycall#115 at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:80 [inlined]
 [11] pycall at /home/takafumi/.julia/dev/PyCall/src/pyfncall.jl:80 [inlined]
 [12] showable(::MIME{Symbol("text/html")}, ::PyObject) at /home/takafumi/.julia/dev/PyCall/src/PyCall.jl:901
 [13] showable(::String, ::Any) at ./multimedia.jl:77
 [14] top-level scope at /home/takafumi/.julia/dev/PyCall/src/pyeval.jl:232

The error is due to invoking Issue816._repr_html_(). A simple solution seems to just return false from showable("text/html", o) when o is a python type. Although it's probably technically possible to implement _repr_html_ on classes as well, it's very likely virtually nobody uses it.

[stevengj]

Can we use PyCallable_Check instead?

[tkf]

Issue816._repr_html_ in the OP is callable so I'm not sure if PyCallable_Check is enough. But maybe PyMethod_Check would work? I'll check.

[tkf]

I considered a few approaches. I think the best way to do this to port IPython.utils.dir2.get_real_method to Julia (or we can just copy dir2.py but it doesn't seem like the traditional approach that PyCall.jl takes). I don't think it covers all edge cases I had in mind (e.g., descriptors) but we don't really need to support edge cases than IPython does. Since Python classes with special _repr_*_ methods are likely to be tested with IPython, following IPython's logic sounds like the best approach.

I think this is good to go.

(Side notes: 95ee1f1 implements one of the alternative methods by using inspect.signature. I think it's a bit more accurate than IPython's get_real_method. However, it's much more complicated. I also considered "easier to ask for forgiveness than permission" approach. But I think it's better to stick with IPython's approach.)

[stevengj]

LGTM — get_real_method is a bit wacky but Python is messy at this level.