Allow overriding constraints with '-o' in constraints file.

news/8076.feature.rst

@@ -0,0 +1 @@
+Allow overriding constraints with ``-o`` in constraints file.

src/pip/_internal/req/constructors.py

@@ -208,6 +208,7 @@ def install_req_from_editable(
     user_supplied: bool = False,
     permit_editable_wheels: bool = False,
     config_settings: Optional[Dict[str, str]] = None,
+    is_override: Optional[bool] = False,
 ) -> InstallRequirement:
 
     parts = parse_req_from_editable(editable_req)
@@ -227,6 +228,7 @@ def install_req_from_editable(
         hash_options=options.get("hashes", {}) if options else {},
         config_settings=config_settings,
         extras=parts.extras,
+        is_override=is_override,
     )
 
 
@@ -383,6 +385,7 @@ def install_req_from_line(
     line_source: Optional[str] = None,
     user_supplied: bool = False,
     config_settings: Optional[Dict[str, str]] = None,
+    is_override: Optional[bool] = False,
 ) -> InstallRequirement:
     """Creates an InstallRequirement from a name, which might be a
     requirement, directory containing 'setup.py', filename, or URL.
@@ -406,6 +409,7 @@ def install_req_from_line(
         constraint=constraint,
         extras=parts.extras,
         user_supplied=user_supplied,
+        is_override=is_override,
     )
 
 
@@ -465,6 +469,7 @@ def install_req_from_parsed_requirement(
             isolated=isolated,
             user_supplied=user_supplied,
             config_settings=config_settings,
+            is_override=parsed_req.is_override,
         )
 
     else:
@@ -478,6 +483,7 @@ def install_req_from_parsed_requirement(
             line_source=parsed_req.line_source,
             user_supplied=user_supplied,
             config_settings=config_settings,
+            is_override=parsed_req.is_override,
         )
     return req
 

src/pip/_internal/req/req_file.py

@@ -87,13 +87,15 @@ def __init__(
         constraint: bool,
         options: Optional[Dict[str, Any]] = None,
         line_source: Optional[str] = None,
+        is_override: Optional[bool] = False,
     ) -> None:
         self.requirement = requirement
         self.is_editable = is_editable
         self.comes_from = comes_from
         self.options = options
         self.constraint = constraint
         self.line_source = line_source
+        self.is_override = is_override
 
 
 class ParsedLine:
@@ -109,6 +111,7 @@ def __init__(
         self.lineno = lineno
         self.opts = opts
         self.constraint = constraint
+        self.is_override = opts.is_override
 
         if args:
             self.is_requirement = True
@@ -200,6 +203,7 @@ def handle_requirement_line(
             constraint=line.constraint,
             options=req_options,
             line_source=line_source,
+            is_override=line.is_override,
         )
 
 
@@ -391,6 +395,13 @@ def parse_line(line: str) -> Tuple[str, Values]:
         if finder:
             defaults.format_control = finder.format_control
 
+        tokens = line.strip().split(" ")
+        if tokens[0] in ("-o", "--override"):
+            is_override = True
+            # Only consider the rest of the line
+            line = " ".join(tokens[1:])
+        else:
+            is_override = False
         args_str, options_str = break_args_options(line)
 
         try:
@@ -399,6 +410,7 @@ def parse_line(line: str) -> Tuple[str, Values]:
             raise OptionParsingError(f"Could not split options: {options_str}") from e
 
         opts, _ = parser.parse_args(options, defaults)
+        opts.is_override = is_override
 
         return args_str, opts
 

src/pip/_internal/req/req_install.py

@@ -91,6 +91,7 @@ def __init__(
         extras: Collection[str] = (),
         user_supplied: bool = False,
         permit_editable_wheels: bool = False,
+        is_override: Optional[bool] = False,
     ) -> None:
         assert req is None or isinstance(req, Requirement), req
         self.req = req
@@ -99,6 +100,7 @@ def __init__(
         self.editable = editable
         self.permit_editable_wheels = permit_editable_wheels
         self.legacy_install_reason: Optional[LegacyInstallReason] = None
+        self.is_override = is_override
 
         # source_dir is the local directory where the linked requirement is
         # located, or unpacked. In case unpacking is needed, creating and

src/pip/_internal/resolution/resolvelib/factory.py

@@ -483,7 +483,7 @@ def collect_root_requirements(
                     continue
                 assert ireq.name, "Constraint must be named"
                 name = canonicalize_name(ireq.name)
-                if name in collected.constraints:
+                if name in collected.constraints and not ireq.is_override:
                     collected.constraints[name] &= ireq
                 else:
                     collected.constraints[name] = Constraint.from_ireq(ireq)