"""
Commonly useful converters.
"""
from __future__ import absolute_import, division, print_function
from ._compat import PY2
from ._make import NOTHING, Factory, pipe
if not PY2:
import inspect
import typing
__all__ = [
"pipe",
"optional",
"default_if_none",
]
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.
:param callable converter: the converter that is used for non-``None``
values.
.. versionadded:: 17.1.0
"""
def optional_converter(val):
if val is None:
return None
return converter(val)
if not PY2:
sig = None
try:
sig = inspect.signature(converter)
except (ValueError, TypeError): # inspect failed
pass
if sig:
params = list(sig.parameters.values())
if params and params[0].annotation is not inspect.Parameter.empty:
optional_converter.__annotations__["val"] = typing.Optional[
params[0].annotation
]
if sig.return_annotation is not inspect.Signature.empty:
optional_converter.__annotations__["return"] = typing.Optional[
sig.return_annotation
]
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*.
:param default: Value to be used if ``None`` is passed. Passing an instance
of `attr.Factory` is supported, however the ``takes_self`` option
is *not*.
:param callable factory: A callable that takes no parameters whose result
is used if ``None`` is passed.
:raises TypeError: If **neither** *default* or *factory* is passed.
:raises TypeError: If **both** *default* and *factory* are passed.
:raises ValueError: If an instance of `attr.Factory` is passed with
``takes_self=True``.
.. versionadded:: 18.2.0
"""
if default is NOTHING and factory is None:
raise TypeError("Must pass either `default` or `factory`.")
if default is not NOTHING and factory is not None:
raise TypeError(
"Must pass either `default` or `factory` but not both."
)
if factory is not None:
default = Factory(factory)
if isinstance(default, Factory):
if default.takes_self:
raise ValueError(
"`takes_self` is not supported by default_if_none."
)
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
