bpo-20523: pdb searches for .pdbrc in ~ instead of $HOME (GH-11847)

Previously pdb checked the $HOME environmental variable
to find the user .pdbrc. If $HOME is not set, the user
.pdbrc would not be found.

Change pdb to use `os.path.expanduser('~')` to determine
the user's home directory. Thus, if $HOME is not set (as
in tox or on Windows), os.path.expanduser('~') falls
back on other techniques for locating the user's home
directory.

This follows pip's implementation for loading .piprc.

Co-authored-by: Dan Lidral-Porter <dlp@aperiodic.org>
(cherry picked from commit 7ea9a85f13)

Co-authored-by: Timothy Hopper <tdhopper@users.noreply.github.com>

Lib/pdb.py

@@ -160,13 +160,11 @@ class Pdb(bdb.Bdb, cmd.Cmd):
         self.allow_kbdint = False
         self.nosigint = nosigint
 
-        # Read $HOME/.pdbrc and ./.pdbrc
+        # Read ~/.pdbrc and ./.pdbrc
         self.rcLines = []
         if readrc:
-            if 'HOME' in os.environ:
-                envHome = os.environ['HOME']
             try:
-                    with open(os.path.join(envHome, ".pdbrc")) as rcFile:
+                with open(os.path.expanduser('~/.pdbrc')) as rcFile:
                     self.rcLines.extend(rcFile)
             except OSError:
                 pass

Lib/test/test_pdb.py

@@ -1377,6 +1377,19 @@ class PdbTestCase(unittest.TestCase):
             if save_home is not None:
                 os.environ['HOME'] = save_home
 
+    def test_readrc_homedir(self):
+        save_home = os.environ.pop("HOME", None)
+        with support.temp_dir() as temp_dir, patch("os.path.expanduser"):
+            rc_path = os.path.join(temp_dir, ".pdbrc")
+            os.path.expanduser.return_value = rc_path
+            try:
+                with open(rc_path, "w") as f:
+                    f.write("invalid")
+                self.assertEqual(pdb.Pdb().rcLines[0], "invalid")
+            finally:
+                if save_home is not None:
+                    os.environ["HOME"] = save_home
+
     def test_header(self):
         stdout = StringIO()
         header = 'Nobody expects... blah, blah, blah'

Misc/NEWS.d/next/Core and Builtins/2019-02-15-20-42-36.bpo-20523.rRLrvr.rst

@@ -0,0 +1,2 @@
+``pdb.Pdb`` supports ~/.pdbrc in Windows 7. Patch by Tim Hopper and Dan 
+Lidral-Porter.