diff --git a/rule_pyflakes.go b/rule_pyflakes.go
index 2f6e665a7..5e4b62719 100644
--- a/rule_pyflakes.go
+++ b/rule_pyflakes.go
@@ -35,15 +35,8 @@ type RulePyflakes struct {
 	mu                    sync.Mutex
 }
 
-// NewRulePyflakes creates new RulePyflakes instance. Parameter executable can be command name
-// or relative/absolute file path. When the given executable is not found in system, it returns
-// an error.
-func NewRulePyflakes(executable string, proc *concurrentProcess) (*RulePyflakes, error) {
-	cmd, err := proc.newCommandRunner(executable)
-	if err != nil {
-		return nil, err
-	}
-	r := &RulePyflakes{
+func newRulePyflakes(cmd *externalCommand) *RulePyflakes {
+	return &RulePyflakes{
 		RuleBase: RuleBase{
 			name: "pyflakes",
 			desc: "Checks for Python script when \"shell: python\" is configured using Pyflakes",
@@ -52,7 +45,17 @@ func NewRulePyflakes(executable string, proc *concurrentProcess) (*RulePyflakes,
 		workflowShellIsPython: shellIsPythonKindUnspecified,
 		jobShellIsPython:      shellIsPythonKindUnspecified,
 	}
-	return r, nil
+}
+
+// NewRulePyflakes creates new RulePyflakes instance. Parameter executable can be command name
+// or relative/absolute file path. When the given executable is not found in system, it returns
+// an error.
+func NewRulePyflakes(executable string, proc *concurrentProcess) (*RulePyflakes, error) {
+	cmd, err := proc.newCommandRunner(executable)
+	if err != nil {
+		return nil, err
+	}
+	return newRulePyflakes(cmd), nil
 }
 
 // VisitJobPre is callback when visiting Job node before visiting its children.
diff --git a/rule_pyflakes_test.go b/rule_pyflakes_test.go
new file mode 100644
index 000000000..e2555d446
--- /dev/null
+++ b/rule_pyflakes_test.go
@@ -0,0 +1,84 @@
+package actionlint
+
+import (
+	"testing"
+)
+
+func TestRulePyflakesDetectPythonShell(t *testing.T) {
+	tests := []struct {
+		what     string
+		isPython bool
+		workflow string // Shell name set at 'defaults' in Workflow node
+		job      string // Shell name set at 'defaults' in Job node
+		step     string // Shell name set at 'shell' in Step node
+	}{
+		{
+			what: "no default shell",
+			isPython: false,
+		},
+		{
+			what: "workflow default",
+			isPython: true,
+			workflow: "python",
+		},
+		{
+			what: "job default",
+			isPython: true,
+			job: "python",
+		},
+		{
+			what: "step shell",
+			isPython: true,
+			step: "python",
+		},
+		{
+			what: "custom shell",
+			isPython: true,
+			workflow: "python {0}",
+		},
+		{
+			what: "other shell",
+			isPython: false,
+			workflow: "pwsh",
+		},
+		{
+			what: "other custom shell",
+			isPython: false,
+			workflow: "bash -e {0}",
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.what, func(t *testing.T) {
+			r := newRulePyflakes(&externalCommand{})
+
+			w := &Workflow{}
+			if tc.workflow != "" {
+				w.Defaults = &Defaults{
+					Run: &DefaultsRun{
+						Shell: &String{Value: tc.workflow},
+					},
+				}
+			}
+			r.VisitWorkflowPre(w)
+
+			j := &Job{}
+			if tc.job != "" {
+				j.Defaults = &Defaults{
+					Run: &DefaultsRun{
+						Shell: &String{Value: tc.job},
+					},
+				}
+			}
+			r.VisitJobPre(j)
+
+			e := &ExecRun{}
+			if tc.step != "" {
+				e.Shell = &String{Value: tc.step}
+			}
+			if have := r.isPythonShell(e); have != tc.isPython {
+				t.Fatalf("Actual isPython=%v but wanted isPython=%v", have, tc.isPython)
+			}
+		})
+	}
+}