fix #1150: enable if setuptools_scm is part of the projects own build-requires

Author: RonnyPfannschmidt

CHANGELOG.md

@@ -15,6 +15,7 @@
 - improve typing for entry_points
 - refactor file modification time logic into shared helper function for better maintainability
 - reduce complexity of HgWorkdir.get_meta method by extracting focused helper methods
+- fix #1150: enable setuptools-scm when we are a build requirement
 
 ### Fixed
 

src/setuptools_scm/_config.py

@@ -14,6 +14,7 @@
 
 from . import _log
 from . import _types as _t
+from ._integration.pyproject_reading import PyProjectData
 from ._integration.pyproject_reading import (
     get_args_for_pyproject as _get_args_for_pyproject,
 )
@@ -115,17 +116,31 @@ def from_file(
         cls,
         name: str | os.PathLike[str] = "pyproject.toml",
         dist_name: str | None = None,
-        _require_section: bool = True,
+        missing_file_ok: bool = False,
         **kwargs: Any,
     ) -> Configuration:
         """
-        Read Configuration from pyproject.toml (or similar).
-        Raises exceptions when file is not found or toml is
-        not installed or the file has invalid format or does
-        not contain the [tool.setuptools_scm] section.
+                Read Configuration from pyproject.toml (or similar).
+                Raises exceptions when file is not found or toml is
+                not installed or the file has invalid format or does
+                not contain setuptools_scm configuration (either via
+        _        [tool.setuptools_scm] section or build-system.requires).
         """
 
-        pyproject_data = _read_pyproject(Path(name), require_section=_require_section)
+        try:
+            pyproject_data = _read_pyproject(Path(name))
+        except FileNotFoundError:
+            if missing_file_ok:
+                log.warning("File %s not found, using empty configuration", name)
+                pyproject_data = PyProjectData(
+                    path=Path(name),
+                    tool_name="setuptools_scm",
+                    project={},
+                    section={},
+                    is_required=False,
+                )
+            else:
+                raise
         args = _get_args_for_pyproject(pyproject_data, dist_name, kwargs)
 
         args.update(read_toml_overrides(args["dist_name"]))

src/setuptools_scm/_integration/pyproject_reading.py

@@ -4,6 +4,7 @@
 
 from pathlib import Path
 from typing import NamedTuple
+from typing import Sequence
 
 from .. import _log
 from .setuptools import read_dist_name_from_setup_cfg
@@ -20,30 +21,56 @@ class PyProjectData(NamedTuple):
     tool_name: str
     project: TOML_RESULT
     section: TOML_RESULT
+    is_required: bool
 
     @property
     def project_name(self) -> str | None:
         return self.project.get("name")
 
 
+def has_build_package(
+    requires: Sequence[str], build_package_names: Sequence[str]
+) -> bool:
+    for requirement in requires:
+        import re
+
+        # Remove extras like [toml] first
+        clean_req = re.sub(r"\[.*?\]", "", requirement)
+        # Split on version operators and take first part
+        package_name = re.split(r"[><=!~]", clean_req)[0].strip().lower()
+        if package_name in build_package_names:
+            return True
+    return False
+
+
 def read_pyproject(
     path: Path = Path("pyproject.toml"),
     tool_name: str = "setuptools_scm",
-    require_section: bool = True,
+    build_package_names: Sequence[str] = ("setuptools_scm", "setuptools-scm"),
 ) -> PyProjectData:
-    defn = read_toml_content(path, None if require_section else {})
+    defn = read_toml_content(path)
+    requires: list[str] = defn.get("build-system", {}).get("requires", [])
+    is_required = has_build_package(requires, build_package_names)
+
     try:
         section = defn.get("tool", {})[tool_name]
     except LookupError as e:
-        error = f"{path} does not contain a tool.{tool_name} section"
-        if require_section:
+        if not is_required:
+            # Enhanced error message that mentions both configuration options
+            error = (
+                f"{path} does not contain a tool.{tool_name} section. "
+                f"setuptools_scm requires configuration via either:\n"
+                f"  1. [tool.{tool_name}] section in {path}, or\n"
+                f"  2. {tool_name} (or setuptools-scm) in [build-system] requires"
+            )
             raise LookupError(error) from e
         else:
+            error = f"{path} does not contain a tool.{tool_name} section"
             log.warning("toml section missing %r", error, exc_info=True)
             section = {}
 
     project = defn.get("project", {})
-    return PyProjectData(path, tool_name, project, section)
+    return PyProjectData(path, tool_name, project, section, is_required)
 
 
 def get_args_for_pyproject(

src/setuptools_scm/_integration/setuptools.py

@@ -45,6 +45,25 @@ def _warn_on_old_setuptools(_version: str = setuptools.__version__) -> None:
         )
 
 
+def _extract_package_name(requirement: str) -> str:
+    """Extract the package name from a requirement string.
+
+    Examples:
+        'setuptools_scm' -> 'setuptools_scm'
+        'setuptools-scm>=8' -> 'setuptools-scm'
+        'setuptools_scm[toml]>=7.0' -> 'setuptools_scm'
+    """
+    # Split on common requirement operators and take the first part
+    # This handles: >=, <=, ==, !=, >, <, ~=
+    import re
+
+    # Remove extras like [toml] first
+    requirement = re.sub(r"\[.*?\]", "", requirement)
+    # Split on version operators
+    package_name = re.split(r"[><=!~]", requirement)[0].strip()
+    return package_name
+
+
 def _assign_version(
     dist: setuptools.Distribution, config: _config.Configuration
 ) -> None:
@@ -97,7 +116,7 @@ def version_keyword(
 
     config = _config.Configuration.from_file(
         dist_name=dist_name,
-        _require_section=False,
+        missing_file_ok=True,
         **overrides,
     )
     _assign_version(dist, config)
@@ -115,6 +134,7 @@ def infer_version(dist: setuptools.Distribution) -> None:
         return
     if dist_name == "setuptools-scm":
         return
+
     try:
         config = _config.Configuration.from_file(dist_name=dist_name)
     except LookupError as e: