fix: cog restores the current directory

Without this fix, if the user code changed the directory, then cog
wouldn't be able to write the output file.

Author: nedbat

CHANGELOG.rst

@@ -21,6 +21,9 @@ These are changes to Cog over time.
 Unreleased
 ----------
 
+- Embedded code can change the current directory, cog will change back to the
+  original directory when the code is done.
+
 - Dropped support for Python 3.7 and 3.8, and added 3.13.
 
 

cogapp/cogapp.py

@@ -413,6 +413,8 @@ def process_file(self, file_in, file_out, fname=None, globals=None):
             file_name_out = file_out
             file_out = file_out_to_close = self.open_output_file(file_out)
 
+        start_dir = os.getcwd()
+
         try:
             file_in = NumberedFileReader(file_in)
 
@@ -592,6 +594,7 @@ def process_file(self, file_in, file_out, fname=None, globals=None):
                 file_in_to_close.close()
             if file_out_to_close:
                 file_out_to_close.close()
+            os.chdir(start_dir)
 
     # A regex for non-empty lines, used by suffixLines.
     re_non_empty_lines = re.compile(r"^\s*\S+.*$", re.MULTILINE)

cogapp/test_cogapp.py

@@ -1409,6 +1409,37 @@ def test_verbosity2(self):
             output, "Cogging unchanged.cog\nCogging changed.cog  (changed)\n"
         )
 
+    def test_change_dir(self):
+        # The code can change directories, cog will move us back.
+        d = {
+            "sub": {
+                "data.txt": "Hello!",
+            },
+            "test.cog": """\
+                //[[[cog
+                import os
+                os.chdir("sub")
+                cog.outl(open("data.txt").read())
+                //]]]
+                //[[[end]]]
+                """,
+            "test.out": """\
+                //[[[cog
+                import os
+                os.chdir("sub")
+                cog.outl(open("data.txt").read())
+                //]]]
+                Hello!
+                //[[[end]]]
+                """,
+        }
+
+        make_files(d)
+        self.cog.callable_main(["argv0", "-r", "test.cog"])
+        self.assertFilesSame("test.cog", "test.out")
+        output = self.output.getvalue()
+        self.assertIn("(changed)", output)
+
 
 class CogTestLineEndings(TestCaseWithTempDir):
     """Tests for -U option (force LF line-endings in output)."""