Python: Better python version error detection

python/lib/dependabot/python/update_checker/pip_compile_version_resolver.rb

@@ -172,14 +172,10 @@ def run_pip_compile_command(command)
           run_command("pyenv local #{python_version}")
           run_command(command)
         rescue SharedHelpers::HelperSubprocessFailed => e
-          original_error ||= e
+          original_err ||= e
           msg = e.message
 
-          relevant_error =
-            if error_suggests_bad_python_version?(msg) then original_error
-            else e
-            end
-
+          relevant_error = choose_relevant_error(original_err, e)
           raise relevant_error unless error_suggests_bad_python_version?(msg)
           raise relevant_error if user_specified_python_version
           raise relevant_error if python_version == "2.7.16"
@@ -191,6 +187,25 @@ def run_pip_compile_command(command)
           FileUtils.remove_entry(".python-version", true)
         end
 
+        def choose_relevant_error(previous_error, new_error)
+          return previous_error if previous_error == new_error
+
+          # If the previous error was definitely due to using the wrong Python
+          # version, return the new error (which can't be worse)
+          if error_certainly_bad_python_version?(previous_error.message)
+            return new_error
+          end
+
+          # Otherwise, if the new error may be due to using the wrong Python
+          # version, return the old error (which can't be worse)
+          if error_suggests_bad_python_version?(new_error.message)
+            return previous_error
+          end
+
+          # Otherwise, default to the new error
+          new_error
+        end
+
         def python_env
           env = {}
 
@@ -206,6 +221,14 @@ def python_env
           env
         end
 
+        def error_certainly_bad_python_version?(message)
+          unless message.include?('Command "python setup.py egg_info" failed')
+            return false
+          end
+
+          message.include?("SyntaxError")
+        end
+
         def error_suggests_bad_python_version?(message)
           return true if message.include?("not find a version that satisfies")
           return true if message.include?("No matching distribution found")

python/spec/dependabot/python/update_checker/pip_compile_version_resolver_spec.rb

@@ -342,6 +342,24 @@
         it { is_expected.to eq(Gem::Version.new("1.6.6")) }
       end
 
+      context "that has swapped syntax in the latest setup.py" do
+        let(:manifest_fixture_name) { "legacy_python_3.in" }
+        let(:generated_fixture_name) { "pip_compile_legacy_python_3.txt" }
+
+        let(:dependency_name) { "django-adv-cache-tag" }
+        let(:dependency_version) { "0.2.1" }
+        let(:dependency_requirements) do
+          [{
+            file: "requirements/test.in",
+            requirement: "==0.2.1",
+            groups: [],
+            source: nil
+          }]
+        end
+
+        it { is_expected.to be_nil }
+      end
+
       context "that has a .python-version file" do
         let(:dependency_files) do
           [manifest_file, generated_file, python_version_file]

python/spec/fixtures/pip_compile_files/legacy_python_3.in

@@ -0,0 +1,2 @@
+oauth2==1.5.211
+django-adv-cache-tag==0.2.1

python/spec/fixtures/requirements/pip_compile_legacy_python_3.txt

@@ -0,0 +1,9 @@
+#
+# This file is autogenerated by pip-compile
+# To update, run:
+#
+#    pip-compile requirements.in
+#
+django-adv-cache-tag==0.2.1
+httplib2==0.12.3          # via oauth2
+oauth2==1.5.211