Prettyerrors

[jw3126]

On master the following snippet:

using PyCall
py"""
def raise_multi_line_error():
    raise Exception("1\n2\n3\n4\n")
"""
py"raise_multi_line_error"()

gives

ERROR: LoadError: PyError ($(Expr(:escape, :(ccall(#= /home/jan/.julia/dev/PyCall/src/pyfncall.jl:43
 =# @pysym(:PyObject_Call), PyPtr, (PyPtr, PyPtr, PyPtr), o, pyargsptr, kw))))) <class 'Exception'>
Exception('1\n2\n3\n4\n')
  File "/home/jan/.julia/dev/PyCall/src/pyeval.jl", line 2, in raise_multi_line_error
    const Py_file_input = 257

This is hard to read. See also #843 . With this PR it becomes:

ERROR: LoadError: PyError:
1
2
3
4

  File "/home/jan/.julia/dev/PyCall/src/pyeval.jl", line 2, in raise_multi_line_error
    const Py_file_input = 257

[jw3126]

This line is crucial. e.val is the python exception object. And the question is how should we print it. Master does it calling repr. From looking at some tutorials, it seems that calling str is preferred. But I am by no means sure if it is the best analog of showerror in python. Or if it even depends on the python version, what's the preferred thing.

[jw3126]

The code is a bit quick and dirty and there are no tests. I would like to have some feedback about how errors should look like and if this is the right approach, before I clean it up.

[jw3126]

Is there interest in this PR?

[jw3126]

@stevengj can you briefly comment if there is interest in this PR? So I know if I should polish / redesign / abandon it.