chore: use Literal type for valid target values (#227)

* use `Literal` type for valid target values

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* fix missing import

* [pre-commit.ci] auto fixes from pre-commit.com hooks

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* remove `TypeAlias` import since it's not needed and doesn't exist on older python versions

* use `typeing_extensions.Literal` for python 3.7

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* Update src/pdm/backend/base.py

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Frost Ming <[email protected]>

src/pdm/backend/base.py

@@ -7,7 +7,15 @@
 import warnings
 from fnmatch import fnmatch
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Iterable, Mapping, TypeVar, cast
+from typing import (
+    TYPE_CHECKING,
+    Any,
+    Iterable,
+    Literal,
+    Mapping,
+    TypeVar,
+    cast,
+)
 
 from pdm.backend.config import Config
 from pdm.backend.exceptions import PDMWarning, ValidationError
@@ -81,12 +89,15 @@ def _find_top_packages(root: str) -> list[str]:
     return result
 
 
+Target = Literal["sdist", "wheel", "editable"]
+
+
 class Builder:
     """Base class for building and distributing a package from given path."""
 
     DEFAULT_EXCLUDES = [".pdm-build"]
 
-    target: str
+    target: Target
     hooks: list[BuildHookInterface] = [DynamicVersionBuildHook()]
 
     def __init__(

src/pdm/backend/hooks/base.py

@@ -9,7 +9,7 @@
 if TYPE_CHECKING:
     from typing import Protocol
 
-    from pdm.backend.base import Builder
+    from pdm.backend.base import Builder, Target
 else:
     Protocol = object
 
@@ -44,7 +44,7 @@ def root(self) -> Path:
         return self.builder.location
 
     @property
-    def target(self) -> str:
+    def target(self) -> Target:
         """The target to build, one of 'sdist', 'wheel', 'editable'"""
         return self.builder.target