Add canned explanation about feature preview flag

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

news/10165.trivial.rst

@@ -0,0 +1,4 @@
+Add a ``feature_flag`` optional kwarg to the ``deprecated()`` function
+``pip._internal.utils.deprecation:deprecated``. Also formulate a corresponding canned
+message which suggests using the ``--use-feature={feature_flag}`` to test upcoming
+behavior.

src/pip/_internal/utils/deprecation.py

@@ -11,6 +11,15 @@
 from pip import __version__ as current_version
 
 DEPRECATION_MSG_PREFIX = "DEPRECATION: "
+DEPRECATION_MESSAGE = DEPRECATION_MSG_PREFIX + "{reason}"
+GONE_IN_MESSAGE_FUTURE = "pip {gone_in} will enforce this behavior change."
+GONE_IN_MESSAGE_PAST = "This behavior change has been enforced since pip {gone_in}."
+REPLACEMENT_MESSAGE = "A possible replacement is {replacement}."
+FEATURE_FLAG_MESSAGE = (
+    "You can temporarily use the flag --use-feature={feature_flag} "
+    "to test the upcoming behavior."
+)
+ISSUE_MESSAGE = "Discussion can be found at https://github.com/pypa/pip/issues/{issue}."
 
 
 class PipDeprecationWarning(Warning):
@@ -56,49 +65,63 @@ def deprecated(
     reason: str,
     replacement: Optional[str],
     gone_in: Optional[str],
+    feature_flag: Optional[str] = None,
     issue: Optional[int] = None,
 ) -> None:
     """Helper to deprecate existing functionality.
 
     reason:
         Textual reason shown to the user about why this functionality has
-        been deprecated.
+        been deprecated. Should be a complete sentence.
     replacement:
         Textual suggestion shown to the user about what alternative
         functionality they can use.
     gone_in:
         The version of pip does this functionality should get removed in.
-        Raises errors if pip's current version is greater than or equal to
+        Raises an error if pip's current version is greater than or equal to
         this.
+    feature_flag:
+        Command-line flag of the form --use-feature={feature_flag} for testing
+        upcoming functionality.
     issue:
         Issue number on the tracker that would serve as a useful place for
         users to find related discussion and provide feedback.
 
     Always pass replacement, gone_in and issue as keyword arguments for clarity
     at the call site.
     """
-
+    # Determine whether or not the feature is already gone in this version.
+    is_gone = gone_in is not None and parse(current_version) >= parse(gone_in)
+    # Allow variable substitutions within the "reason" variable.
+    formatted_reason = reason.format(gone_in=gone_in)
     # Construct a nice message.
     #   This is eagerly formatted as we want it to get logged as if someone
     #   typed this entire message out.
+    formatted_deprecation_message = DEPRECATION_MESSAGE.format(reason=formatted_reason)
+    gone_in_message = GONE_IN_MESSAGE_PAST if is_gone else GONE_IN_MESSAGE_FUTURE
+    formatted_gone_in_message = (
+        gone_in_message.format(gone_in=gone_in) if gone_in else None
+    )
+    formatted_replacement_message = (
+        REPLACEMENT_MESSAGE.format(replacement=replacement) if replacement else None
+    )
+    formatted_feature_flag_message = (
+        None
+        if is_gone or not feature_flag
+        else FEATURE_FLAG_MESSAGE.format(feature_flag=feature_flag)
+    )
+    formatted_issue_message = ISSUE_MESSAGE.format(issue=issue) if issue else None
     sentences = [
-        (reason, DEPRECATION_MSG_PREFIX + "{}"),
-        (gone_in, "pip {} will remove support for this functionality."),
-        (replacement, "A possible replacement is {}."),
-        (
-            issue,
-            (
-                "You can find discussion regarding this at "
-                "https://github.com/pypa/pip/issues/{}."
-            ),
-        ),
+        formatted_deprecation_message,
+        formatted_gone_in_message,
+        formatted_replacement_message,
+        formatted_feature_flag_message,
+        formatted_issue_message,
     ]
-    message = " ".join(
-        template.format(val) for val, template in sentences if val is not None
-    )
+    message = " ".join(sentence for sentence in sentences if sentence)
 
-    # Raise as an error if it has to be removed.
-    if gone_in is not None and parse(current_version) >= parse(gone_in):
+    # Raise as an error if the functionality is gone.
+    if is_gone:
         raise PipDeprecationWarning(message)
-
-    warnings.warn(message, category=PipDeprecationWarning, stacklevel=2)
+    else:
+        warnings.warn(message, category=PipDeprecationWarning, stacklevel=2)

tests/unit/test_utils.py

@@ -934,9 +934,9 @@ def test_deprecated_message_reads_well():
 
     assert message == (
         "DEPRECATION: Stop doing this! "
-        "pip 1.0 will remove support for this functionality. "
+        "This behavior change has been enforced since pip 1.0. "
         "A possible replacement is to be nicer. "
-        "You can find discussion regarding this at "
+        "Discussion can be found at "
         "https://github.com/pypa/pip/issues/100000."
     )