basic subscriptions system

1 files changed, 78 insertions, 0 deletions

diff --git a/python/attr/converters.py b/python/attr/converters.py
new file mode 100644
index 0000000..37c4a07
--- /dev/null
+++ b/python/attr/converters.py
@@ -0,0 +1,78 @@
+"""
+Commonly useful converters.
+"""
+
+from __future__ import absolute_import, division, print_function
+
+from ._make import NOTHING, Factory
+
+
+def optional(converter):
+    """
+    A converter that allows an attribute to be optional. An optional attribute
+    is one which can be set to ``None``.
+
+    :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)
+
+    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 :class:`attr.Factory` is supported, however the ``takes_self`` option
+       is *not*.
+    :param callable factory: A callable that takes not 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 :class:`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