Make "failed to find application" more informative (#1648)

Co-Authored-By: Luis Liñán Villafranca <luislivilla@gmail.com>

Fixes #1647

gunicorn/util.py

@@ -363,7 +363,7 @@ def import_app(module):
     except NameError:
         if is_debug:
             traceback.print_exception(*sys.exc_info())
-        raise AppImportError("Failed to find application: %r" % module)
+        raise AppImportError("Failed to find application object %r in %r" % (obj, module))
 
     if app is None:
         raise AppImportError("Failed to find application object: %r" % obj)

tests/support.py

@@ -2,10 +2,26 @@ import functools
 import sys
 import unittest
 import platform
+from wsgiref.validate import validator
 
 HOST = "127.0.0.1"
 
 
+@validator
+def app(environ, start_response):
+    """Simplest possible application object"""
+
+    data = b'Hello, World!\n'
+    status = '200 OK'
+
+    response_headers = [
+        ('Content-type', 'text/plain'),
+        ('Content-Length', str(len(data))),
+    ]
+    start_response(status, response_headers)
+    return iter([data])
+
+
 def requires_mac_ver(*min_version):
     """Decorator raising SkipTest if the OS is Mac OS X and the OS X
     version if less than min_version.

tests/test_util.py

@@ -6,6 +6,7 @@
 import pytest
 
 from gunicorn import util
+from gunicorn.errors import AppImportError
 
 
 @pytest.mark.parametrize('test_input, expected', [
@@ -45,3 +46,16 @@ def test_warn(capsys):
     util.warn('test warn')
     _, err = capsys.readouterr()
     assert '!!! WARNING: test warn' in err
+
+
+def test_import_app():
+    assert util.import_app('support:app')
+
+    with pytest.raises(ImportError) as err:
+        util.import_app('a:app')
+    assert 'No module' in str(err)
+
+    with pytest.raises(AppImportError) as err:
+        util.import_app('support:wrong_app')
+    msg = "Failed to find application object 'wrong_app' in 'support'"
+    assert msg in str(err)