Add monadic utilities MonadicDict and MonadicDictCursor.

* wqflask/utility/monads.py: New file.

1 files changed, 80 insertions, 0 deletions

diff --git a/wqflask/utility/monads.py b/wqflask/utility/monads.py
new file mode 100644
index 00000000..fbcb2c23
--- /dev/null
+++ b/wqflask/utility/monads.py
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+"""Monadic utilities
+
+This module is a collection of monadic utilities for use in
+GeneNetwork. It includes:
+
+* MonadicDict - monadic version of the built-in dictionary
+* MonadicDictCursor - monadic version of MySQLdb.cursors.DictCursor
+  that returns a MonadicDict instead of the built-in dictionary
+"""
+
+from collections import UserDict
+from functools import partial
+
+from MySQLdb.cursors import DictCursor
+from pymonad.maybe import Just, Nothing
+
+class MonadicDict(UserDict):
+    """
+    Monadic version of the built-in dictionary.
+
+    Keys in this dictionary can be any python object, but values must
+    be monadic values.
+    """
+    def __init__(self, d, convert=True):
+        """
+        Initialize monadic dictionary.
+
+        If convert is False, values in dictionary d must be
+        monadic. If convert is True, values in dictionary d are
+        converted to monadic values.
+        """
+        if convert:
+            super().__init__({key:(Nothing if value is None else Just(value))
+                              for key, value in d.items()})
+        else:
+            super().__init__(d)
+    def __getitem__(self, key):
+        """
+        Get key from dictionary.
+
+        If key exists in the dictionary, return a Just value. Else,
+        return Nothing.
+        """
+        try:
+            return Just(self.data[key])
+        except KeyError:
+            return Nothing
+    def __setitem__(self, key, value):
+        """
+        Set key in dictionary.
+
+        value must be a monadic value---either Nothing or a Just
+        value. If value is a Just value, set it in the dictionary. If
+        value is Nothing, do nothing.
+        """
+        value.bind(partial(super().__setitem__, key))
+    def __delitem__(self, key):
+        """
+        Delete key from dictionary.
+
+        If key exists in the dictionary, delete it. Else, do nothing.
+        """
+        try:
+            super().__delitem__(key)
+        except KeyError:
+            pass
+
+class MonadicDictCursor(DictCursor):
+    """
+    Monadic version of MySQLdb.cursors.DictCursor.
+
+    Monadic version of MySQLdb.cursors.DictCursor that returns a
+    MonadicDict instead of the built-in dictionary.
+    """
+    def fetchone(self):
+        return MonadicDict(super().fetchone())
+    def fetchmany(self, size=None):
+        return [MonadicDict(row) for row in super().fetchmany(size=size)]
+    def fetchall(self):
+        return [MonadicDict(row) for row in super().fetchall()]