Only read pyproject.toml if file exists

Author: rgaiacs

repo2docker/buildpacks/pyproject/__init__.py

@@ -135,14 +135,15 @@ def detect(self):
             return False
 
         pyproject_file = self.binder_path("pyproject.toml")
-        with open(pyproject_file, "rb") as _pyproject_file:
-            pyproject_toml = tomllib.load(_pyproject_file)
-
-        if (
-            ("project" in pyproject_toml)
-            and ("build-system" in pyproject_toml)
-            and ("requires" in pyproject_toml["build-system"])
-        ):
-            return os.path.exists(pyproject_file)
+        if os.path.exists(pyproject_file):
+            with open(pyproject_file, "rb") as _pyproject_file:
+                pyproject_toml = tomllib.load(_pyproject_file)
+
+            if (
+                ("project" in pyproject_toml)
+                and ("build-system" in pyproject_toml)
+                and ("requires" in pyproject_toml["build-system"])
+            ):
+                return os.path.exists(pyproject_file)
 
         return False