Added debughelpers. Flask will now tell you if you forget enctype

CHANGES

@@ -16,6 +16,8 @@ Relase date to be decided, codename to be chosen.
   OPTIONS implementation.
 - HTTP exceptions and Bad Request Key Errors can now be trapped so that they
   show up normally in the traceback.
+- Flask in debug mode is now detecting some common problems and tries to
+  warn you about them.
 
 Version 0.7.3
 -------------

flask/debughelpers.py

@@ -0,0 +1,52 @@
+# -*- coding: utf-8 -*-
+"""
+    flask.debughelpers
+    ~~~~~~~~~~~~~~~~~~
+
+    Various helpers to make the development experience better.
+
+    :copyright: (c) 2011 by Armin Ronacher.
+    :license: BSD, see LICENSE for more details.
+"""
+
+
+class DebugFilesKeyError(KeyError, AssertionError):
+    """Raised from request.files during debugging.  The idea is that it can
+    provide a better error message than just a generic KeyError/BadRequest.
+    """
+
+    def __init__(self, request, key):
+        form_matches = request.form.getlist(key)
+        buf = ['You tried to access the file "%s" in the request.files '
+               'dictionary but it does not exist.  The mimetype for the request '
+               'is "%s" instead of "multipart/form-data" which means that no '
+               'files were transmitted.  To fix this error you most likely have '
+               'to provide enctype="multipart/form-data" in your form.' %
+               (key, request.mimetype)]
+        if form_matches:
+            buf.append('\n\nThe browser instead most likely submitted the '
+                       'filenames in the form.  This was submitted: %s' %
+                       ', '.join('"%s"' % x for x in form_matches))
+        self.msg = ''.join(buf)
+
+    def __str__(self):
+        return self.msg
+
+
+def make_enctype_error_multidict(request):
+    """Since Flask 0.8 we're monkeypatching the files object in case a
+    request is detected that does not use multipart form data but the files
+    object is accessed.
+    """
+    oldcls = request.files.__class__
+    class newcls(oldcls):
+        def __getitem__(self, key):
+            try:
+                return oldcls.__getitem__(self, key)
+            except KeyError, e:
+                if key not in request.form:
+                    raise
+                raise DebugFilesKeyError(request, key)
+    newcls.__name__ = oldcls.__name__
+    newcls.__module__ = oldcls.__module__
+    request.files.__class__ = newcls

flask/wrappers.py

@@ -12,6 +12,7 @@
 from werkzeug.wrappers import Request as RequestBase, Response as ResponseBase
 from werkzeug.utils import cached_property
 
+from .debughelpers import make_enctype_error_multidict
 from .helpers import json, _assert_have_json
 from .globals import _request_ctx_stack
 
@@ -99,6 +100,16 @@ class Request(RequestBase):
                 return json.loads(self.data, encoding=request_charset)
             return json.loads(self.data)
 
+    def _load_form_data(self):
+        RequestBase._load_form_data(self)
+
+        # in debug mode we're replacing the files multidict with an ad-hoc
+        # subclass that raises a different error for key errors.
+        ctx = _request_ctx_stack.top
+        if ctx is not None and ctx.app.debug and \
+           self.mimetype != 'multipart/form-data':
+            make_enctype_error_multidict(self)
+
 
 class Response(ResponseBase):
     """The response object that is used by default in Flask.  Works like the