Lowering error with global default value for a parameter

A=[1., 2.]
@numba.njit
def f(f_values=A):
    return 0   
f()

results in something which is hard to interpret to me (below). At the same time

f(A)

works fine, which suggests that this could be easily fixed.

---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/targets/base.py in get_constant_generic(self, builder, ty, val)
    453         try:
--> 454             impl = self._get_constants.find((ty,))
    455             return impl(self, builder, ty, val)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/targets/base.py in find(self, sig)
     47         if out is None:
---> 48             out = self._find(sig)
     49             self._cache[sig] = out

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/targets/base.py in _find(self, sig)
     56         else:
---> 57             raise NotImplementedError(self, sig)
     58 

NotImplementedError: (<numba.targets.base.OverloadSelector object at 0x7ffa086aa320>, (reflected list(float64),))

During handling of the above exception, another exception occurred:

NotImplementedError                       Traceback (most recent call last)
~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/errors.py in new_error_context(fmt_, *args, **kwargs)
    258     try:
--> 259         yield
    260     except NumbaError as e:

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_block(self, block)
    215                                    loc=self.loc, errcls_=defaulterrcls):
--> 216                 self.lower_inst(inst)
    217 

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_inst(self, inst)
    264             ty = self.typeof(inst.target.name)
--> 265             val = self.lower_assign(ty, inst)
    266             self.storevar(val, inst.target.name)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_assign(self, ty, inst)
    428                 const = self.context.get_constant_generic(self.builder, valty,
--> 429                                                           pyval)
    430                 # cast it to the variable type

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/targets/base.py in get_constant_generic(self, builder, ty, val)
    456         except NotImplementedError:
--> 457             raise NotImplementedError("cannot lower constant of type '%s'" % (ty,))
    458 

NotImplementedError: cannot lower constant of type 'reflected list(float64)'

During handling of the above exception, another exception occurred:

LoweringError                             Traceback (most recent call last)
<ipython-input-18-a5b8ed516f38> in <module>()
      4     return 0
      5 
----> 6 f()

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/dispatcher.py in _compile_for_args(self, *args, **kws)
    305                 argtypes.append(self.typeof_pyval(a))
    306         try:
--> 307             return self.compile(tuple(argtypes))
    308         except errors.TypingError as e:
    309             # Intercept typing error that may be due to an argument

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/dispatcher.py in compile(self, sig)
    577 
    578                 self._cache_misses[sig] += 1
--> 579                 cres = self._compiler.compile(args, return_type)
    580                 self.add_overload(cres)
    581                 self._cache.save_overload(sig, cres)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/dispatcher.py in compile(self, args, return_type)
     78                                       impl,
     79                                       args=args, return_type=return_type,
---> 80                                       flags=flags, locals=self.locals)
     81         # Check typing error if object mode is used
     82         if cres.typing_error is not None and not flags.enable_pyobject:

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in compile_extra(typingctx, targetctx, func, args, return_type, flags, locals, library)
    777     pipeline = Pipeline(typingctx, targetctx, library,
    778                         args, return_type, flags, locals)
--> 779     return pipeline.compile_extra(func)
    780 
    781 

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in compile_extra(self, func)
    360         self.lifted = ()
    361         self.lifted_from = None
--> 362         return self._compile_bytecode()
    363 
    364     def compile_ir(self, func_ir, lifted=(), lifted_from=None):

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in _compile_bytecode(self)
    736         """
    737         assert self.func_ir is None
--> 738         return self._compile_core()
    739 
    740     def _compile_ir(self):

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in _compile_core(self)
    723 
    724         pm.finalize()
--> 725         res = pm.run(self.status)
    726         if res is not None:
    727             # Early pipeline completion

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in run(self, status)
    246                     # No more fallback pipelines?
    247                     if is_final_pipeline:
--> 248                         raise patched_exception
    249                     # Go to next fallback pipeline
    250                     else:

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in run(self, status)
    238                 try:
    239                     event(stage_name)
--> 240                     stage()
    241                 except _EarlyPipelineCompletion as e:
    242                     return e.result

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in stage_nopython_backend(self)
    656         """
    657         lowerfn = self.backend_nopython_mode
--> 658         self._backend(lowerfn, objectmode=False)
    659 
    660     def stage_compile_interp_mode(self):

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in _backend(self, lowerfn, objectmode)
    611             self.library.enable_object_caching()
    612 
--> 613         lowered = lowerfn()
    614         signature = typing.signature(self.return_type, *self.args)
    615         self.cr = compile_result(typing_context=self.typingctx,

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in backend_nopython_mode(self)
    598                 self.return_type,
    599                 self.calltypes,
--> 600                 self.flags)
    601 
    602     def _backend(self, lowerfn, objectmode):

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/compiler.py in native_lowering_stage(targetctx, library, interp, typemap, restype, calltypes, flags)
    896 
    897     lower = lowering.Lower(targetctx, library, fndesc, interp)
--> 898     lower.lower()
    899     if not flags.no_cpython_wrapper:
    900         lower.create_cpython_wrapper(flags.release_gil)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower(self)
    133         if self.generator_info is None:
    134             self.genlower = None
--> 135             self.lower_normal_function(self.fndesc)
    136         else:
    137             self.genlower = self.GeneratorLower(self)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_normal_function(self, fndesc)
    174         # Init argument values
    175         self.extract_function_arguments()
--> 176         entry_block_tail = self.lower_function_body()
    177 
    178         # Close tail of entry block

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_function_body(self)
    199             bb = self.blkmap[offset]
    200             self.builder.position_at_end(bb)
--> 201             self.lower_block(block)
    202 
    203         self.post_lower()

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_block(self, block)
    214             with new_error_context('lowering "{inst}" at {loc}', inst=inst,
    215                                    loc=self.loc, errcls_=defaulterrcls):
--> 216                 self.lower_inst(inst)
    217 
    218     def create_cpython_wrapper(self, release_gil=False):

~/miniconda3/envs/emmy/lib/python3.6/contextlib.py in __exit__(self, type, value, traceback)
     97                 value = type()
     98             try:
---> 99                 self.gen.throw(type, value, traceback)
    100             except StopIteration as exc:
    101                 # Suppress StopIteration *unless* it's the same exception that

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/errors.py in new_error_context(fmt_, *args, **kwargs)
    263     except Exception as e:
    264         newerr = errcls(e).add_context(_format_msg(fmt_, args, kwargs))
--> 265         six.reraise(type(newerr), newerr, sys.exc_info()[2])
    266 
    267 

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/six.py in reraise(tp, value, tb)
    656             value = tp()
    657         if value.__traceback__ is not tb:
--> 658             raise value.with_traceback(tb)
    659         raise value
    660 

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/errors.py in new_error_context(fmt_, *args, **kwargs)
    257 
    258     try:
--> 259         yield
    260     except NumbaError as e:
    261         e.add_context(_format_msg(fmt_, args, kwargs))

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_block(self, block)
    214             with new_error_context('lowering "{inst}" at {loc}', inst=inst,
    215                                    loc=self.loc, errcls_=defaulterrcls):
--> 216                 self.lower_inst(inst)
    217 
    218     def create_cpython_wrapper(self, release_gil=False):

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_inst(self, inst)
    263         if isinstance(inst, ir.Assign):
    264             ty = self.typeof(inst.target.name)
--> 265             val = self.lower_assign(ty, inst)
    266             self.storevar(val, inst.target.name)
    267 

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/lowering.py in lower_assign(self, ty, inst)
    427                 valty = self.context.typing_context.resolve_value_type(pyval)
    428                 const = self.context.get_constant_generic(self.builder, valty,
--> 429                                                           pyval)
    430                 # cast it to the variable type
    431                 res = self.context.cast(self.builder, const, valty, ty)

~/miniconda3/envs/emmy/lib/python3.6/site-packages/numba/targets/base.py in get_constant_generic(self, builder, ty, val)
    455             return impl(self, builder, ty, val)
    456         except NotImplementedError:
--> 457             raise NotImplementedError("cannot lower constant of type '%s'" % (ty,))
    458 
    459     def get_constant(self, ty, val):

LoweringError: Failed at nopython (nopython mode backend)
cannot lower constant of type 'reflected list(float64)'
File "<ipython-input-18-a5b8ed516f38>", line 4
[1] During: lowering "f_values = arg(0, name=f_values)" at <ipython-input-18-a5b8ed516f38> (4)

[seibert]

The “reflected” part is referring to the fact that a list that comes into a Numba function as an argument needs to have any changes in contents of the nopython mode version of that list reflected in the original Python list object when the function exits. Tuples do not have this complexity in Numba (and generally work more places) because they are immutable.

(Incidentally, this is also why using a list as a default argument value is typically avoided in Python. It is very easy for one call to the function to accidentally mutate the default value and affect future calls to the function.)

[stuartarchibald]

I expect this is because Numba is treating A in f_values=A as a compile-time-constant as it is a global http://numba.pydata.org/numba-doc/latest/glossary.html#term-compile-time-constant. However there is no mechanism written in Numba for lowering (translating from the higher level representation to a lower level) a reflected list as a constant type. If you make A into a constant type that can be lowered, like a homogeneous tuple or a scalar then compilation will succeed.