From 00b4388245f7777276af351e3afd68af2417c426 Mon Sep 17 00:00:00 2001
From: Matthew Dapena-Tretter <m@tthewwithanm.com>
Date: Tue, 23 Sep 2014 18:35:38 -0400
Subject: [PATCH] Support should_verify_existence on strategies
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This prevents extra IO. Different defaults are used for async backends
since we can’t assume that `existence_required` resulted in existence
synchronously.
---
 imagekit/cachefiles/__init__.py   | 9 ++++++++-
 imagekit/cachefiles/backends.py   | 2 ++
 imagekit/cachefiles/strategies.py | 3 +++
 tests/utils.py                    | 2 ++
 4 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/imagekit/cachefiles/__init__.py b/imagekit/cachefiles/__init__.py
index 34892ec..2ba5cb8 100644
--- a/imagekit/cachefiles/__init__.py
+++ b/imagekit/cachefiles/__init__.py
@@ -128,7 +128,14 @@ class ImageCacheFile(BaseIKFile, ImageFile):
         # Dispatch the existence_required signal before checking to see if the
         # file exists. This gives the strategy a chance to create the file.
         existence_required.send(sender=self, file=self)
-        return self.cachefile_backend.exists(self)
+
+        try:
+            check = self.cachefile_strategy.should_verify_existence(self)
+        except AttributeError:
+            # All synchronous backends should have created the file as part of
+            # `existence_required` if they wanted to.
+            check = getattr(self.cachefile_backend, 'is_async', False)
+        return self.cachefile_backend.exists(self) if check else True
 
     def __getstate__(self):
         state = copy(self.__dict__)
diff --git a/imagekit/cachefiles/backends.py b/imagekit/cachefiles/backends.py
index 25ce43f..1512f51 100644
--- a/imagekit/cachefiles/backends.py
+++ b/imagekit/cachefiles/backends.py
@@ -121,6 +121,8 @@ class BaseAsync(Simple):
     """
     Base class for cache file backends that generate files asynchronously.
     """
+    is_async = True
+
     def generate(self, file, force=False):
         # Schedule the file for generation, unless we know for sure we don't
         # need to. If an already-generated file sneaks through, that's okay;
diff --git a/imagekit/cachefiles/strategies.py b/imagekit/cachefiles/strategies.py
index 98f712e..bcd8211 100644
--- a/imagekit/cachefiles/strategies.py
+++ b/imagekit/cachefiles/strategies.py
@@ -29,6 +29,9 @@ class Optimistic(object):
     def on_source_saved(self, file):
         file.generate()
 
+    def should_verify_existence(self, file):
+        return False
+
 
 class DictStrategy(object):
     def __init__(self, callbacks):
diff --git a/tests/utils.py b/tests/utils.py
index 5a9f1c5..b6c4952 100644
--- a/tests/utils.py
+++ b/tests/utils.py
@@ -77,5 +77,7 @@ class DummyAsyncCacheFileBackend(Simple):
     A cache file backend meant to simulate async generation.
 
     """
+    is_async = True
+
     def generate(self, file, force=False):
         pass