[RFC] Throw a special exceptions in Julia callbacks (Fix #770) #772

PhilipVinc · 2020-05-01T13:20:26Z

This PR tries to address the printing of stack traces in Julia callbacks and fix #770, while also leaving the door open for further improvements.

The problem I wanted to solve is the fact that errors and stack traces thrown by a Julia callback executed by python, such as

julia> py"$error('I am ugly')"

loses formatting when being displayed.
This is because currently we render the exception to a string when it is thrown, return it to python as a python error, which then is returned to Julia again, and displays it.

This is especially painful in wrapper packages such as Tensorflow.jl, Jax.jl or several others.

In the process formatting is lost.

My solution:
When a Julia callback throws, don't render the callback immediately. Instead return a custom Error type, PyJlError, which stores the backtrace.

When control is eventually returned to Julia, check if the python error is wrapping a Julia error. If so, show only the Julia one.

This is compatible with JuliaPy, as the rendering of PyJlError is handled by the show method.

Result:
old:

julia> using PyCall
julia> py"$error('I am ugly')"
ERROR: PyError ($(Expr(:escape, :(ccall(#= /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:38 =# @pysym(:PyEval_EvalCode), PyPtr, (PyPtr, PyPtr, PyPtr), o, globals, locals))))) <class 'RuntimeError'>
RuntimeError('Julia exception: I am ugly\nStacktrace:\n [1] error(::String) at ./error.jl:33\n [2] #invokelatest#1 at ./essentials.jl:712 [inlined]\n [3] invokelatest(::Any, ::Any) at ./essentials.jl:711\n [4] _pyjlwrap_call(::Function, ::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}) at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/callback.jl:28\n [5] pyjlwrap_call(::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}) at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/callback.jl:49\n [6] macro expansion at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/exception.jl:93 [inlined]\n [7] #120 at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:38 [inlined]\n [8] disable_sigint at ./c.jl:446 [inlined]\n [9] pyeval_(::String, ::PyDict{String,PyObject,true}, ::PyDict{String,PyObject,true}, ::Int64, ::String) at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:37\n [10] top-level scope at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:230\n [11] eval(::Module, ::Any) at ./boot.jl:331\n [12] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86\n [13] run_backend(::REPL.REPLBackend) at /Users/filippovicentini/.julia/packages/Revise/Pcs5V/src/Revise.jl:1073\n [14] top-level scope at none:0\n [15] eval(::Module, ::Any) at ./boot.jl:331\n [16] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86\n [17] macro expansion at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:118 [inlined]\n [18] (::REPL.var"#26#27"{REPL.REPLBackend})() at ./task.jl:358')
  File "/Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl", line 1, in <module>
    const Py_single_input = 256  # from Python.h

Stacktrace:
 [1] pyerr_check at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/exception.jl:60 [inlined]
 [2] pyerr_check at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/exception.jl:64 [inlined]
 [3] _handle_error(::String) at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/exception.jl:81
 [4] macro expansion at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/exception.jl:95 [inlined]
 [5] #120 at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:38 [inlined]
 [6] disable_sigint at ./c.jl:446 [inlined]
 [7] pyeval_(::String, ::PyDict{String,PyObject,true}, ::PyDict{String,PyObject,true}, ::Int64, ::String) at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:37
 [8] top-level scope at /Users/filippovicentini/.julia/packages/PyCall/zqDXB/src/pyeval.jl:230
 [9] eval(::Module, ::Any) at ./boot.jl:331
 [10] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
 [11] run_backend(::REPL.REPLBackend) at /Users/filippovicentini/.julia/packages/Revise/Pcs5V/src/Revise.jl:1073
 [12] top-level scope at none:0

new:

julia> py"$error('I am ugly')"
ERROR: An error occured in a Julia function called from Python
I am ugly
Stacktrace:
 [1] error(::String) at ./error.jl:33
 [2] #invokelatest#1 at ./essentials.jl:712 [inlined]
 [3] invokelatest(::Any, ::Any) at ./essentials.jl:711
 [4] _pyjlwrap_call(::Function, ::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}) at /Users/filippovicentini/.julia/dev/PyCall/src/callback.jl:28
 [5] pyjlwrap_call(::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}, ::Ptr{PyCall.PyObject_struct}) at /Users/filippovicentini/.julia/dev/PyCall/src/callback.jl:49
 [6] macro expansion at /Users/filippovicentini/.julia/dev/PyCall/src/exception.jl:117 [inlined]
 [7] #120 at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:38 [inlined]
 [8] disable_sigint at ./c.jl:446 [inlined]
 [9] pyeval_(::String, ::PyDict{String,PyObject,true}, ::PyDict{String,PyObject,true}, ::Int64, ::String) at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:37
 [10] top-level scope at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:230
 [11] eval(::Module, ::Any) at ./boot.jl:331
 [12] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
 [13] run_backend(::REPL.REPLBackend) at /Users/filippovicentini/.julia/packages/Revise/Pcs5V/src/Revise.jl:1073
 [14] top-level scope at none:0
 [15] eval(::Module, ::Any) at ./boot.jl:331
 [16] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
 [17] macro expansion at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:118 [inlined]
 [18] (::REPL.var"#26#27"{REPL.REPLBackend})() at ./task.jl:358
Stacktrace:
 [1] pyerr_check at /Users/filippovicentini/.julia/dev/PyCall/src/exception.jl:84 [inlined]
 [2] pyerr_check at /Users/filippovicentini/.julia/dev/PyCall/src/exception.jl:88 [inlined]
 [3] _handle_error(::String) at /Users/filippovicentini/.julia/dev/PyCall/src/exception.jl:105
 [4] macro expansion at /Users/filippovicentini/.julia/dev/PyCall/src/exception.jl:119 [inlined]
 [5] #120 at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:38 [inlined]
 [6] disable_sigint at ./c.jl:446 [inlined]
 [7] pyeval_(::String, ::PyDict{String,PyObject,true}, ::PyDict{String,PyObject,true}, ::Int64, ::String) at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:37
 [8] top-level scope at /Users/filippovicentini/.julia/dev/PyCall/src/pyeval.jl:230
 [9] eval(::Module, ::Any) at ./boot.jl:331
 [10] eval_user_input(::Any, ::REPL.REPLBackend) at /Users/julia/buildbot/worker/package_macos64/build/usr/share/julia/stdlib/v1.4/REPL/src/REPL.jl:86
 [11] run_backend(::REPL.REPLBackend) at /Users/filippovicentini/.julia/packages/Revise/Pcs5V/src/Revise.jl:1073
 [12] top-level scope at none:0

src/exception.jl

PhilipVinc · 2020-05-02T16:15:52Z

So...
one error is because in this line

        if p != NULL # for dict, set, etc.

NULL is not defined.

The other one is the one with Unprintable and I'm on it...

PhilipVinc · 2020-05-02T17:09:15Z

Ok, the second error is... very weird.

In short, the solution is removing the parametric type from PyJlError, which is not really needed anyway.

However, I don't understand why that is the case. So below is a small investigation on the cause. Maybe it's an underlying issue in Julia, or maybe in my implementation?

I tracked it down to exactly this line

    err = PyJlError(e, bt)

The error also persists if I try to do

t=typeof(e) #executes ok
err = PyJlError{t}(e, bt)

In case it might help, this error still happens if you make Unprintable's type printable but not the value.

stevengj · 2020-05-02T17:32:20Z

I think if you wanted to keep the parametric type you would need an outer constructor PyJlError(err::E, trace) where {E} = PyJlError{E}(err, trace)?

But I agree that a parameterized type is not needed here.

stevengj · 2020-05-02T17:33:24Z

The error also persists if I try to do t=typeof(e); err = PyJlError{e}(e, bt)

I think you mean err = PyJlError{t}(e, bt)?

stevengj · 2020-05-02T17:33:37Z

But let's leave the type as non-parametric.

PhilipVinc · 2020-05-02T17:46:54Z

(Now tests are passing, except for an unrelated error which I'm not sure why shows up half the time).

I think you mean err = PyJlError{t}(e, bt)?

Yes, indeed.
I think I understood what the problem is: I just found out that in Julia you can throw any object as an error.

On of the tests tries throwing an Unprintable object (which is not <:Exception) and so it could not build a PyJlerror which was throwing again.. and so on.

stevengj · 2020-05-02T18:30:16Z

@tkf, hopefully this doesn't cause any problems for pyjulia?

tkf · 2020-05-03T04:22:08Z

Thanks for the ping. I ran PyJulia test suite with PyCall master and there was no problem. I looked at the patch and played with it a bit. So far it works nicely.