Merge pull request #13 from carver/lazy-auto-backend

Auto import with no backend is now supported

eth_hash/auto.py

@@ -1,8 +1,8 @@
+from eth_hash.backends.auto import (
+    AutoBackend,
+)
 from eth_hash.main import (
     Keccak256,
 )
-from eth_hash.utils import (
-    auto_choose_backend,
-)
 
-keccak = Keccak256(auto_choose_backend())
+keccak = Keccak256(AutoBackend())

eth_hash/backends/auto.py

@@ -0,0 +1,16 @@
+from eth_hash.utils import (
+    auto_choose_backend,
+)
+
+
+class AutoBackend:
+    _backend = None
+
+    @property
+    def backend(self):
+        if self._backend is None:
+            self._backend = auto_choose_backend()
+        return self._backend
+
+    def __getattr__(self, attr):
+        return getattr(self.backend, attr)

eth_hash/main.py

@@ -9,10 +9,25 @@
 
 class Keccak256:
     def __init__(self, backend):
-        self.hasher = backend.keccak256
-        self.preimage = backend.preimage
+        self._backend = backend
+        self.hasher = self._hasher_first_run
+        self.preimage = self._preimage_first_run
 
-        assert self.hasher(b'') == b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';\x7b\xfa\xd8\x04]\x85\xa4p"  # noqa: E501
+    def _hasher_first_run(self, preimage):
+        '''
+        Invoke the backend on-demand, and check an expected hash result,
+        then replace this first run with the new hasher method.
+        This is a bit of a hacky way to minimize overhead on hash calls after this first one.
+        '''
+        new_hasher = self._backend.keccak256
+        assert new_hasher(b'') == b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';\x7b\xfa\xd8\x04]\x85\xa4p"  # noqa: E501
+        self.hasher = new_hasher
+        return new_hasher(preimage)
+
+    def _preimage_first_run(self, data):
+        new_preimage = self._backend.preimage
+        self.preimage = new_preimage
+        return new_preimage(data)
 
     def __call__(self, preimage: Union[bytes, bytearray]) -> bytes:
         if not isinstance(preimage, (bytes, bytearray)):

tests/core/test_import.py

@@ -1,6 +1,16 @@
 import pytest
 
 
+def test_import_auto():
+    from eth_hash.auto import keccak  # noqa: F401
+
+
+def test_import_auto_empty_crash():
+    from eth_hash.auto import keccak
+    with pytest.raises(ImportError, match="None of these hashing backends are installed"):
+        keccak(b'')
+
+
 def test_import():
     import eth_hash  # noqa: F401
 
@@ -13,9 +23,10 @@ def test_import():
     ],
 )
 def test_load_by_env(monkeypatch, backend):
+    from eth_hash.auto import keccak
     monkeypatch.setenv('ETH_HASH_BACKEND', backend)
     with pytest.raises(ImportError) as excinfo:
-        from eth_hash.auto import keccak  # noqa: F401
+        keccak(b'triggered')
     expected_msg = (
         "The backend specified in ETH_HASH_BACKEND, '{0}', is not installed. "
         "Install with `pip install eth-hash[{0}]`.".format(backend)
@@ -24,10 +35,11 @@ def test_load_by_env(monkeypatch, backend):
 
 
 def test_load_unavailable_backend_by_env(monkeypatch):
+    from eth_hash.auto import keccak
     backend = 'this-backend-will-never-exist'
     monkeypatch.setenv('ETH_HASH_BACKEND', backend)
     with pytest.raises(ValueError) as excinfo:
-        from eth_hash.auto import keccak  # noqa: F401
+        keccak(b'triggered')
     expected_msg = (
         "The backend specified in ETH_HASH_BACKEND, '{0}', is not supported. "
         "Choose one of".format(backend)