python: expand and refactor tests

This brings coverage of `Language::Python::Shebang` to 100%.

Library/Homebrew/test/language/python/shebang_spec.rb

@@ -5,17 +5,31 @@ require "utils/shebang"
 
 describe Language::Python::Shebang do
   let(:file) { Tempfile.new("python-shebang") }
-  let(:python_f) do
-    formula "python@3.11" do
+  let(:f) do
+    f = {}
+
+    f[:python311] = formula "python@3.11" do
       url "https://brew.sh/python-1.0.tgz"
     end
-  end
-  let(:f) do
-    formula "foo" do
+
+    f[:versioned_python_dep] = formula "foo" do
       url "https://brew.sh/foo-1.0.tgz"
 
       depends_on "python@3.11"
     end
+
+    f[:no_deps] = formula "foo" do
+      url "https://brew.sh/foo-1.0.tgz"
+    end
+
+    f[:multiple_deps] = formula "foo" do
+      url "https://brew.sh/foo-1.0.tgz"
+
+      depends_on "python"
+      depends_on "python@3.11"
+    end
+
+    f
   end
 
   before do
@@ -33,9 +47,9 @@ describe Language::Python::Shebang do
   describe "#detected_python_shebang" do
     it "can be used to replace Python shebangs" do
       allow(Formulary).to receive(:factory)
-      allow(Formulary).to receive(:factory).with(python_f.name).and_return(python_f)
+      allow(Formulary).to receive(:factory).with(f[:python311].name).and_return(f[:python311])
       Utils::Shebang.rewrite_shebang(
-        described_class.detected_python_shebang(f, use_python_from_path: false), file.path
+        described_class.detected_python_shebang(f[:versioned_python_dep], use_python_from_path: false), file.path
       )
 
       expect(File.read(file)).to eq <<~EOS
@@ -48,7 +62,7 @@ describe Language::Python::Shebang do
 
     it "can be pointed to a `python3` in PATH" do
       Utils::Shebang.rewrite_shebang(
-        described_class.detected_python_shebang(f, use_python_from_path: true), file.path
+        described_class.detected_python_shebang(f[:versioned_python_dep], use_python_from_path: true), file.path
       )
 
       expect(File.read(file)).to eq <<~EOS
@@ -58,5 +72,26 @@ describe Language::Python::Shebang do
         c
       EOS
     end
+
+    it "errors if formula doesn't depend on python" do
+      expect do
+        Utils::Shebang.rewrite_shebang(
+          described_class.detected_python_shebang(f[:no_deps], use_python_from_path: false),
+          file.path,
+        )
+      end.to raise_error(ShebangDetectionError, "Cannot detect Python shebang: formula does not depend on Python.")
+    end
+
+    it "errors if formula depends on more than one python" do
+      expect do
+        Utils::Shebang.rewrite_shebang(
+          described_class.detected_python_shebang(f[:multiple_deps], use_python_from_path: false),
+          file.path,
+        )
+      end.to raise_error(
+        ShebangDetectionError,
+        "Cannot detect Python shebang: formula has multiple Python dependencies.",
+      )
+    end
   end
 end