Fix dropped leading characters c from constraints' packages (#3250)

`lstrip` takes a set, not a string, so `.lstrip("-c ")` did not work as
intended.

Until we only support Python 3.9 or higher, we need to use a custom
function to remove a prefix. With Python 3.9+ we can use the builtin
`.removeprefix`.

This fixes #3247.

Co-authored-by: Bernát Gábor <gaborjbernat@gmail.com>

docs/changelog/3247.bugfix.rst

@@ -0,0 +1 @@
+Fix issue that the leading character ``c`` was dropped from packages in constraints files - by :user:`jugmac00`.

src/tox/tox_env/python/pip/pip_install.py

@@ -107,7 +107,7 @@ def constrain_package_deps(self) -> bool:
     def use_frozen_constraints(self) -> bool:
         return bool(self._env.conf["use_frozen_constraints"])
 
-    def _install_requirement_file(self, arguments: PythonDeps, section: str, of_type: str) -> None:
+    def _install_requirement_file(self, arguments: PythonDeps, section: str, of_type: str) -> None:  # noqa: C901
         try:
             new_options, new_reqs = arguments.unroll()
         except ValueError as exception:
@@ -145,7 +145,15 @@ def _install_requirement_file(self, arguments: PythonDeps, section: str, of_type
                 if args:  # pragma: no branch
                     self._execute_installer(args, of_type)
                     if self.constrain_package_deps and not self.use_frozen_constraints:
-                        combined_constraints = new_requirements + [c.lstrip("-c ") for c in new_constraints]
+                        # when we drop Python 3.8 we can use the builtin `.removeprefix`
+                        def remove_prefix(text: str, prefix: str) -> str:
+                            if text.startswith(prefix):
+                                return text[len(prefix) :]
+                            return text
+
+                        combined_constraints = new_requirements + [
+                            remove_prefix(text=c, prefix="-c ") for c in new_constraints
+                        ]
                         self.constraints_file().write_text("\n".join(combined_constraints))
 
     @staticmethod

tests/tox_env/python/pip/test_pip_install.py

@@ -309,8 +309,8 @@ def constrained_mock_project(
         "build.py": (demo_pkg_inline / "build.py").read_text(),
     }
     exp_constraints: list[str] = []
-    requirement = "foo==1.2.3"
-    constraint = "foo<2"
+    requirement = "coo==1.2.3"
+    constraint = "coo<2"
     if request.param == "explicit":
         deps = requirement
         exp_constraints.append(requirement)