Merge pull request #812 from public/circular-fix

Fix circular imports with available_backends()

1 files changed, 26 insertions, 7 deletions

diff --git a/cryptography/hazmat/backends/__init__.py b/cryptography/hazmat/backends/__init__.py
index 59d1bc6c..ae78822c 100644
--- a/cryptography/hazmat/backends/__init__.py
+++ b/cryptography/hazmat/backends/__init__.py
@@ -13,23 +13,42 @@
 
 from __future__ import absolute_import, division, print_function
 
-from cryptography.hazmat.backends import openssl
 from cryptography.hazmat.backends.multibackend import MultiBackend
 from cryptography.hazmat.bindings.commoncrypto.binding import (
     Binding as CommonCryptoBinding
 )
+from cryptography.hazmat.bindings.openssl.binding import (
+    Binding as OpenSSLBinding
+)
+
+
+_available_backends_list = None
+
 
-_ALL_BACKENDS = []
+def _available_backends():
+    global _available_backends_list
 
-if CommonCryptoBinding.is_available():
-    from cryptography.hazmat.backends import commoncrypto
-    _ALL_BACKENDS.append(commoncrypto.backend)
+    if _available_backends_list is None:
+        _available_backends_list = []
 
-_ALL_BACKENDS.append(openssl.backend)
+        if CommonCryptoBinding.is_available():
+            from cryptography.hazmat.backends import commoncrypto
+            _available_backends_list.append(commoncrypto.backend)
 
+        if OpenSSLBinding.is_available():
+            from cryptography.hazmat.backends import openssl
+            _available_backends_list.append(openssl.backend)
 
-_default_backend = MultiBackend(_ALL_BACKENDS)
+    return _available_backends_list
+
+
+_default_backend = None
 
 
 def default_backend():
+    global _default_backend
+
+    if _default_backend is None:
+        _default_backend = MultiBackend(_available_backends())
+
     return _default_backend