Don't require boto3 if Amazon SES webhooks aren't actually used

Delay raising AnymailImproperlyInstalled from webhooks.amazon_ses
until an SES webhook view is instantiated. Allows anymail.urls
to import webhooks.amazon_ses without error.

Fixes #103

anymail/exceptions.py

@@ -177,9 +177,9 @@ class AnymailImproperlyInstalled(AnymailConfigurationError, ImportError):
     """Exception for Anymail missing package dependencies"""
 
     def __init__(self, missing_package, backend="<backend>"):
-        message = "The %s package is required to use this backend, but isn't installed.\n" \
+        message = "The %s package is required to use this ESP, but isn't installed.\n" \
                   "(Be sure to use `pip install django-anymail[%s]` " \
-                  "with your desired backends)" % (missing_package, backend)
+                  "with your desired ESPs.)" % (missing_package, backend)
         super(AnymailImproperlyInstalled, self).__init__(message)
 
 
@@ -195,3 +195,17 @@ class AnymailInsecureWebhookWarning(AnymailWarning):
 
 class AnymailDeprecationWarning(AnymailWarning, DeprecationWarning):
     """Warning for deprecated Anymail features"""
+
+
+# Helpers
+
+class _LazyError(object):
+    """An object that sits inert unless/until used, then raises an error"""
+    def __init__(self, error):
+        self._error = error
+
+    def __call__(self, *args, **kwargs):
+        raise self._error
+
+    def __getattr__(self, item):
+        raise self._error

anymail/webhooks/amazon_ses.py

@@ -6,18 +6,23 @@ from django.http import HttpResponse
 from django.utils.dateparse import parse_datetime
 
 from .base import AnymailBaseWebhookView
-from ..backends.amazon_ses import _get_anymail_boto3_params
 from ..exceptions import (
-    AnymailAPIError, AnymailConfigurationError, AnymailImproperlyInstalled, AnymailWebhookValidationFailure)
+    AnymailAPIError, AnymailConfigurationError, AnymailImproperlyInstalled, AnymailWebhookValidationFailure,
+    _LazyError)
 from ..inbound import AnymailInboundMessage
 from ..signals import AnymailInboundEvent, AnymailTrackingEvent, EventType, RejectReason, inbound, tracking
 from ..utils import combine, get_anymail_setting, getfirst
 
 try:
     import boto3
-    import botocore.exceptions
+    from botocore.exceptions import ClientError
+    from ..backends.amazon_ses import _get_anymail_boto3_params
 except ImportError:
-    raise AnymailImproperlyInstalled(missing_package='boto3', backend='amazon_ses')
+    # This module gets imported by anymail.urls, so don't complain about boto3 missing
+    # unless one of the Amazon SES webhook views is actually used and needs it
+    boto3 = _LazyError(AnymailImproperlyInstalled(missing_package='boto3', backend='amazon_ses'))
+    ClientError = object
+    _get_anymail_boto3_params = _LazyError(AnymailImproperlyInstalled(missing_package='boto3', backend='amazon_ses'))
 
 
 class AmazonSESBaseWebhookView(AnymailBaseWebhookView):
@@ -296,7 +301,7 @@ class AmazonSESInboundWebhookView(AmazonSESBaseWebhookView):
                 s3.download_fileobj(bucket_name, object_key, content)
                 content.seek(0)
                 message = AnymailInboundMessage.parse_raw_mime_file(content)
-            except botocore.exceptions.ClientError as err:
+            except ClientError as err:
                 # improve the botocore error message
                 raise AnymailBotoClientAPIError(
                     "Anymail AmazonSESInboundWebhookView couldn't download S3 object '{bucket_name}:{object_key}'"
@@ -334,11 +339,11 @@ class AmazonSESInboundWebhookView(AmazonSESBaseWebhookView):
         )]
 
 
-class AnymailBotoClientAPIError(AnymailAPIError, botocore.exceptions.ClientError):
+class AnymailBotoClientAPIError(AnymailAPIError, ClientError):
     """An AnymailAPIError that is also a Boto ClientError"""
     def __init__(self, *args, **kwargs):
         raised_from = kwargs.pop('raised_from')
-        assert isinstance(raised_from, botocore.exceptions.ClientError)
+        assert isinstance(raised_from, ClientError)
         assert len(kwargs) == 0  # can't support other kwargs
         # init self as boto ClientError (which doesn't cooperatively subclass):
         super(AnymailBotoClientAPIError, self).__init__(