Make Julia exceptions wrapped in Python Exceptions more informative (#608)

* Make Julia exceptions wrapped in Python Exceptions more informative

I have added a backtrace to the description of exceptions coming from Julia
that are wrapped in Python exceptions.

This makes it much easier to find the source of errors in Julia code being
called from Python functions.

* Allow `pyraise` to include backtrace from Julia

Instead of using `catch_backtrace` in `showerror_string`, which may not always
be called from a catch block with a current exception set, I have added an
optional argument to `pyraise` which can be used to pass backtraces to
`showerror_string`.

Backtraces are only used if exception is not a PyError exception. By default, no
backtrace is shown.

All try-catch blocks in PyCall attempt to pass the backtrace for the current
exception to `pyraise`.

* Add pyraise macro

Added a pyraise macro, as written by @stevengj, which catches backtraces and
calls the pyraise function. This macro should only be used in a catch block.

* Allow ioraise to display Julia backtrace

Author: galenlynch

src/callback.jl

@@ -39,7 +39,7 @@ function _pyjlwrap_call(f, args_::PyPtr, kw_::PyPtr)
 
         return pyreturn(ret)
     catch e
-        pyraise(e)
+        @pyraise e
     finally
         args.o = PyPtr_NULL # don't decref
     end

src/exception.jl

@@ -123,16 +123,21 @@ function pyexc_initialize()
     pyexc[PyIOError] = @pyglobalobjptr :PyExc_IOError
 end
 
+_showerror_string(io::IO, e, ::Nothing) = showerror(io, e)
+_showerror_string(io::IO, e, bt) = showerror(io, e, bt)
+
+# bt argument defaults to nothing, to delay dispatching on the presence of a
+# backtrace until after the try-catch block
 """
     showerror_string(e) :: String
 
 Convert output of `showerror` to a `String`.  Since this function may
 be called via Python C-API, it tries to not throw at all cost.
 """
-function showerror_string(e::T) where {T}
+function showerror_string(e::T, bt = nothing) where {T}
     try
         io = IOBuffer()
-        showerror(io, e)
+        _showerror_string(io, e, bt)
         return String(take!(io))
     catch
         try
@@ -163,15 +168,26 @@ function showerror_string(e::T) where {T}
     end
 end
 
-function pyraise(e)
+function pyraise(e, bt = nothing)
     eT = typeof(e)
     pyeT = haskey(pyexc::Dict, eT) ? pyexc[eT] : pyexc[Exception]
     ccall((@pysym :PyErr_SetString), Cvoid, (PyPtr, Cstring),
-          pyeT, string("Julia exception: ", showerror_string(e)))
+          pyeT, string("Julia exception: ", showerror_string(e, bt)))
 end
 
-function pyraise(e::PyError)
+# Second argument allows for backtraces passed to `pyraise` to be ignored.
+function pyraise(e::PyError, ::Vector = [])
     ccall((@pysym :PyErr_Restore), Cvoid, (PyPtr, PyPtr, PyPtr),
           e.T, e.val, e.traceback)
     e.T.o = e.val.o = e.traceback.o = C_NULL # refs were stolen
 end
+
+"""
+    @pyraise e
+
+Throw the exception `e` to Python, attaching a backtrace.  This macro should only be
+used in a `catch` block so that `catch_backtrace()` is valid.
+"""
+macro pyraise(e)
+    :(pyraise($(esc(e)), catch_backtrace()))
+end

src/io.jl

@@ -8,21 +8,21 @@
 # IOBase methods:
 
 # IO objects should raise IOError for unsupported operations or failed IO
-function ioraise(e)
+function ioraise(e, bt = nothing)
     if isa(e, MethodError) || isa(e, ErrorException)
         ccall((@pysym :PyErr_SetString), Cvoid, (PyPtr, Cstring),
               (pyexc::Dict)[PyIOError],
-              string("Julia exception: ", e))
+              showerror_string(e, bt))
     else
-        pyraise(e)
+        pyraise(e, bt)
     end
 end
 
 macro with_ioraise(expr)
     :(try
         $(esc(expr))
       catch e
-        ioraise(e)
+        ioraise(e, catch_backtrace())
       end)
 end
 

src/pyiterator.jl

@@ -134,7 +134,7 @@ const jlWrapIteratorType = PyTypeObject()
                 return pyreturn(item)
             end
         catch e
-            pyraise(e)
+            @pyraise e
         end
         return PyPtr_NULL
     end
@@ -149,7 +149,7 @@ else
                 return pyreturn(item)
             end
         catch e
-            pyraise(e)
+            @pyraise e
         end
         return PyPtr_NULL
     end
@@ -162,7 +162,7 @@ function pyjlwrap_getiter(self_::PyPtr)
         self = unsafe_pyjlwrap_to_objref(self_)
         return pystealref!(jlwrap_iterator(self))
     catch e
-        pyraise(e)
+        @pyraise e
     end
     return PyPtr_NULL
 end

src/pytype.jl

@@ -348,7 +348,7 @@ function pyjlwrap_repr(o::PyPtr)
         return pyreturn(o != C_NULL ? string("<PyCall.jlwrap ",unsafe_pyjlwrap_to_objref(o),">")
                         : "<PyCall.jlwrap NULL>")
     catch e
-        pyraise(e)
+        @pyraise e
         return PyPtr_NULL
     end
 end
@@ -395,7 +395,7 @@ function pyjlwrap_getattr(self_::PyPtr, attr__::PyPtr)
             end
         end
     catch e
-        pyraise(e)
+        @pyraise e
     finally
         attr_.o = PyPtr_NULL # don't decref
     end