feat: fallback_version config for scm version

Signed-off-by: Frost Ming <me@frostming.com>

docs/metadata.md

@@ -44,6 +44,15 @@ When building from a source tree where SCM is not available, you can use the env
 PDM_BUILD_SCM_VERSION=0.1.0 python -m build
 ```
 
++++ 2.2.0
+
+Alternatively, you can specify a default version in the configuration:
+
+```toml
+[tool.pdm.version]
+fallback_version = "0.0.0"
+```
+
 You can specify another regex pattern to match the SCM tag, in which a `version` group is required:
 
 ```toml

src/pdm/backend/hooks/version/__init__.py

@@ -76,6 +76,7 @@ def resolve_version_from_scm(
         write_template: str = "{}\n",
         tag_regex: str | None = None,
         version_format: str | None = None,
+        fallback_version: str | None = None,
     ) -> str:
         if "PDM_BUILD_SCM_VERSION" in os.environ:
             version = os.environ["PDM_BUILD_SCM_VERSION"]
@@ -89,6 +90,15 @@ def resolve_version_from_scm(
             version = get_version_from_scm(
                 context.root, tag_regex=tag_regex, version_formatter=version_formatter
             )
+            if version is None:
+                if fallback_version is not None:
+                    version = fallback_version
+                else:
+                    raise ConfigError(
+                        "Cannot find the version from SCM or SCM isn't detected. \n"
+                        "You can still specify the version via environment variable "
+                        "`PDM_BUILD_SCM_VERSION`, or specify `fallback_version` config."
+                    )
 
         self._write_version(context, version, write_to, write_template)
         return version

src/pdm/backend/hooks/version/scm.py

@@ -333,16 +333,12 @@ def get_version_from_scm(
     *,
     tag_regex: str | None = None,
     version_formatter: Callable[[SCMVersion], str] | None = None,
-) -> str:
+) -> str | None:
     config = Config(tag_regex=re.compile(tag_regex) if tag_regex else DEFAULT_TAG_REGEX)
     for func in (git_parse_version, hg_parse_version):
         version = func(root, config)  # type: ignore
         if version:
             if version_formatter is None:
                 version_formatter = format_version
             return version_formatter(version)
-    raise ValueError(
-        "Cannot find the version from SCM or SCM isn't detected. \n"
-        "You can still specify the version via environment variable "
-        "`PDM_BUILD_SCM_VERSION`."
-    )
+    return None