Usage of a lambda function as the request callback

README.md

@@ -10,5 +10,6 @@ A Flake8 plugin to catch common issues on Scrapy spiders.
 | SCP02 | There are URLs in `allowed_domains` |
 | SCP03 | Usage of `urljoin(response.url, '/foo')` instead of `response.urljoin('/foo')` |
 | SCP04 | Usage of `Selector(response)` in callback |
+| SCP05 | Usage of a lambda function as the request callback |
 
 This is a work in progress, so new issues will be added to this list.

finders/unsupported.py

@@ -0,0 +1,40 @@
+import ast
+
+from finders import IssueFinder
+
+
+class LambdaCallbackIssueFinder(IssueFinder):
+    msg_code = 'SCP05'
+    msg_info = 'callback should not be a lambda'
+
+    def is_scrapy_request(self, node):
+        return (
+            isinstance(node.func, ast.Attribute) and
+            node.func.value.id == 'scrapy' and
+            node.func.attr == 'Request'
+        )
+
+    def is_raw_request(self, node):
+        return (
+            isinstance(node.func, ast.Name) and
+            node.func.id == 'Request'
+        )
+
+    def issue_applies(self, node):
+        return (
+            self.is_raw_request(node) or
+            self.is_scrapy_request(node)
+        )
+
+    def find_issues(self, node):
+        if not self.issue_applies(node):
+            return
+
+        if len(node.args) >= 2:
+            callback = node.args[1]
+            if isinstance(callback, ast.Lambda):
+                return [(callback.lineno, callback.col_offset, self.message)]
+
+        for kw in node.keywords:
+            if kw.arg == 'callback' and isinstance(kw.value, ast.Lambda):
+                return [(node.lineno, node.col_offset, self.message)]

flake8_scrapy.py

@@ -4,7 +4,7 @@
     UnreachableDomainIssueFinder, UrlInAllowedDomainsIssueFinder,
 )
 from finders.oldstyle import OldSelectorIssueFinder, UrlJoinIssueFinder
-
+from finders.unsupported import LambdaCallbackIssueFinder
 
 __version__ = '0.0.1'
 
@@ -22,6 +22,7 @@ def __init__(self, *args, **kwargs):
             ],
             'Call': [
                 UrlJoinIssueFinder(),
+                LambdaCallbackIssueFinder(),
             ]
         }
 

tests/test_unsupported.py

@@ -0,0 +1,16 @@
+import pytest
+
+from . import run_checker
+
+
+@pytest.mark.parametrize('code,expected', [
+    ('Request(x, callback=lambda x: x)', 1),
+    ('scrapy.Request(x, callback=lambda x: x)', 1),
+    ('scrapy.Request(x, lambda x: x)', 1),
+    ('Request(x, callback=self.parse)', 0),
+    ('scrapy.Request(x, callback=self.parse)', 0),
+    ('scrapy.Request(x, self.parse)', 0),
+])
+def test_find_lambda_callback_issue(code, expected):
+    issues = run_checker(code)
+    assert len(issues) == expected