Return a better error message if a file: URL is not found (pypa#10263)

Co-authored-by: Tzu-ping Chung <uranusjr@gmail.com>
Co-authored-by: Pradyun Gedam <pradyunsg@gmail.com>

news/10263.bugfix.rst

@@ -0,0 +1 @@
+Present a better error message, when a ``file:`` URL is not found.

src/pip/_internal/network/session.py

@@ -3,6 +3,7 @@
 """
 
 import email.utils
+import io
 import ipaddress
 import json
 import logging
@@ -207,8 +208,11 @@ def send(
         try:
             stats = os.stat(pathname)
         except OSError as exc:
+            # format the exception raised as a io.BytesIO object,
+            # to return a better error message:
             resp.status_code = 404
-            resp.raw = exc
+            resp.reason = type(exc).__name__
+            resp.raw = io.BytesIO(f"{resp.reason}: {exc}".encode("utf8"))
         else:
             modified = email.utils.formatdate(stats.st_mtime, usegmt=True)
             content_type = mimetypes.guess_type(pathname)[0] or "text/plain"

tests/functional/test_bad_url.py

@@ -0,0 +1,16 @@
+# test the error message returned by pip when
+# a bad "file:" URL is passed to it.
+
+from typing import Any
+
+
+def test_filenotfound_error_message(script: Any) -> None:
+    # Test the error message returned when using a bad 'file:' URL.
+    # make pip to fail and get an error message
+    # by running "pip install -r file:nonexistent_file"
+    proc = script.pip("install", "-r", "file:unexistent_file", expect_error=True)
+    assert proc.returncode == 1
+    expect = (
+        "ERROR: 404 Client Error: FileNotFoundError for url: file:///unexistent_file"
+    )
+    assert proc.stderr.rstrip() == expect