diff --git a/cytoolz/dicttoolz.pyx b/cytoolz/dicttoolz.pyx
index 1a85532..67a83e1 100644
--- a/cytoolz/dicttoolz.pyx
+++ b/cytoolz/dicttoolz.pyx
@@ -9,7 +9,9 @@ from cpython.ref cimport PyObject, Py_DECREF, Py_INCREF, Py_XDECREF
 from cytoolz.cpython cimport PyDict_Next_Compat, PtrIter_Next
 
 from collections import abc
+import collections.abc
 
+from typing_extensions import TypeVar
 
 # cdef aliases to eliminate global lookups
 
@@ -22,6 +24,10 @@ __all__ = ['merge', 'merge_with', 'valmap', 'keymap', 'itemmap', 'valfilter',
            'update_in']
 
 
+K = TypeVar('K')
+V = TypeVar('V')
+
+
 cdef class _iter_mapping:
     """ Keep a handle on the current item to prevent memory clean up too early"""
     def __cinit__(self, object it):
@@ -370,15 +376,42 @@ cpdef object itemfilter(object predicate, object d, object factory=dict):
 
 
 cpdef object assoc(object d, object key, object value, object factory=dict):
-    """
-    Return a new dict with new key value pair
-
-    New dict has d[key] set to value. Does not modify the initial dictionary.
-
-    >>> assoc({'x': 1}, 'x', 2)
-    {'x': 2}
-    >>> assoc({'x': 1}, 'y', 3)   # doctest: +SKIP
-    {'x': 1, 'y': 3}
+    """assoc(d: collections.abc.Mapping[K, V], key: K, value: V, factory: collections.abc.Callable[[], collections.abc.MutableMapping[K, V]] = dict) -> collections.abc.MutableMapping[K, V]
+
+    Create a new `Mapping`[1] with `key` associated with `value`.
+
+    You can use `assoc` (***assoc***iate) to copy `d` (***d***ictionary) to a new `Mapping` created by
+    `factory` and assign `value` to `key`. `assoc` does not change `d`.
+
+    Parameters
+    ----------
+    d : Mapping[K, V]
+        Source `Mapping`.
+    key : K
+        `key` that `assoc` inserts or replaces.
+    value : V
+        `value` that `assoc` assigns to `key`.
+    factory : Callable[[], MutableMapping[K, V]] = dict
+        `Callable` that creates the `MutableMapping`[1] to `return`.
+
+    Returns
+    -------
+    mappingUpdated : MutableMapping[K, V]
+        New `Mapping` with `key` associated to `value`.
+
+    Examples
+    --------
+    >>> assoc({}, 'a', 1)
+    {'a': 1}
+    >>> assoc({'a': 1}, 'a', 3)
+    {'a': 3}
+    >>> assoc({'a': 1}, 'b', 3)
+    {'a': 1, 'b': 3}
+
+    References
+    ----------
+    [1] Python `collections.abc` module
+        https://docs.python.org/3/library/collections.abc.html
     """
     cdef object rv
     rv = factory()
diff --git a/pyproject.toml b/pyproject.toml
index cfb78d1..5758a83 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -64,6 +64,7 @@ classifiers = [
 ]
 dependencies = [
   "toolz >= 0.8.0",
+  "typing_extensions >= 3.5.0",
 ]
 
 [project.optional-dependencies]