raises ConflictingExplicitBindingsError when it shouldn't

[danmilon]

I bumped into a very very weird issue.

#! /usr/bin/env python2.7
import pinject


class FoundationA(object):
    @pinject.inject()
    def __init__(self, arg):
        self.arg = arg


class FoundationB(object):
    @pinject.inject()
    def __init__(self, arg):
        self._arg = arg


class BindingSpec(pinject.BindingSpec):
    def configure(self, bind):
        bind('arg', to_instance=1)

object_graph = pinject.new_object_graph(
    binding_specs=[
        BindingSpec(),
    ],
    only_use_explicit_bindings=True
)

foundation_a = object_graph.provide(FoundationA)
print(foundation_a.arg)

foundation_b = object_graph.provide(FoundationB)
print(foundation_b.arg)

This should print

1
1

Instead it raises ConflictingExplicitBindingsError.

$ python ./t.py 
Traceback (most recent call last):
  File "./t.py", line 25, in <module>
    only_use_explicit_bindings=True
  File "/tmp/venv/lib/python2.7/site-packages/pinject/object_graph.py", line 159, in new_object_graph
    raise e
pinject.errors.ConflictingExplicitBindingsError: multiple explicit bindings for same binding name:
  the binding at ./t.py:5, from the binding name "foundation" (unannotated) to the class __main__.FoundationA at ./t.py:5, in "singleton scope" scope
  the binding at ./t.py:11, from the binding name "foundation" (unannotated) to the class __main__.FoundationB at ./t.py:11, in "singleton scope" scope

Some very weird magic is happening behind the scenes, even though I've set only_use_explicit_bindings=True!

Any idea what might be wrong?