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+# SPDX-License-Identifier: MIT
+
+"""
+Commonly useful converters.
+"""
+
+import typing
+
+from ._compat import _AnnotationExtractor
+from ._make import NOTHING, Converter, Factory, pipe
+
+
+__all__ = [
+ "default_if_none",
+ "optional",
+ "pipe",
+ "to_bool",
+]
+
+
+def optional(converter):
+ """
+ A converter that allows an attribute to be optional. An optional attribute
+ is one which can be set to `None`.
+
+ Type annotations will be inferred from the wrapped converter's, if it has
+ any.
+
+ Args:
+ converter (typing.Callable):
+ the converter that is used for non-`None` values.
+
+ .. versionadded:: 17.1.0
+ """
+
+ if isinstance(converter, Converter):
+
+ def optional_converter(val, inst, field):
+ if val is None:
+ return None
+ return converter(val, inst, field)
+
+ else:
+
+ def optional_converter(val):
+ if val is None:
+ return None
+ return converter(val)
+
+ xtr = _AnnotationExtractor(converter)
+
+ t = xtr.get_first_param_type()
+ if t:
+ optional_converter.__annotations__["val"] = typing.Optional[t]
+
+ rt = xtr.get_return_type()
+ if rt:
+ optional_converter.__annotations__["return"] = typing.Optional[rt]
+
+ if isinstance(converter, Converter):
+ return Converter(optional_converter, takes_self=True, takes_field=True)
+
+ return optional_converter
+
+
+def default_if_none(default=NOTHING, factory=None):
+ """
+ A converter that allows to replace `None` values by *default* or the result
+ of *factory*.
+
+ Args:
+ default:
+ Value to be used if `None` is passed. Passing an instance of
+ `attrs.Factory` is supported, however the ``takes_self`` option is
+ *not*.
+
+ factory (typing.Callable):
+ A callable that takes no parameters whose result is used if `None`
+ is passed.
+
+ Raises:
+ TypeError: If **neither** *default* or *factory* is passed.
+
+ TypeError: If **both** *default* and *factory* are passed.
+
+ ValueError:
+ If an instance of `attrs.Factory` is passed with
+ ``takes_self=True``.
+
+ .. versionadded:: 18.2.0
+ """
+ if default is NOTHING and factory is None:
+ msg = "Must pass either `default` or `factory`."
+ raise TypeError(msg)
+
+ if default is not NOTHING and factory is not None:
+ msg = "Must pass either `default` or `factory` but not both."
+ raise TypeError(msg)
+
+ if factory is not None:
+ default = Factory(factory)
+
+ if isinstance(default, Factory):
+ if default.takes_self:
+ msg = "`takes_self` is not supported by default_if_none."
+ raise ValueError(msg)
+
+ def default_if_none_converter(val):
+ if val is not None:
+ return val
+
+ return default.factory()
+
+ else:
+
+ def default_if_none_converter(val):
+ if val is not None:
+ return val
+
+ return default
+
+ return default_if_none_converter
+
+
+def to_bool(val):
+ """
+ Convert "boolean" strings (for example, from environment variables) to real
+ booleans.
+
+ Values mapping to `True`:
+
+ - ``True``
+ - ``"true"`` / ``"t"``
+ - ``"yes"`` / ``"y"``
+ - ``"on"``
+ - ``"1"``
+ - ``1``
+
+ Values mapping to `False`:
+
+ - ``False``
+ - ``"false"`` / ``"f"``
+ - ``"no"`` / ``"n"``
+ - ``"off"``
+ - ``"0"``
+ - ``0``
+
+ Raises:
+ ValueError: For any other value.
+
+ .. versionadded:: 21.3.0
+ """
+ if isinstance(val, str):
+ val = val.lower()
+
+ if val in (True, "true", "t", "yes", "y", "on", "1", 1):
+ return True
+ if val in (False, "false", "f", "no", "n", "off", "0", 0):
+ return False
+
+ msg = f"Cannot convert value to bool: {val!r}"
+ raise ValueError(msg)