diff --git a/swh/core/api/__init__.py b/swh/core/api/__init__.py
--- a/swh/core/api/__init__.py
+++ b/swh/core/api/__init__.py
@@ -17,9 +17,9 @@
                           encode_data_client as encode_data,
                           msgpack_dumps, msgpack_loads, SWHJSONDecoder)
 
-from .negotiate import (Formatter as FormatterBase,
-                        Negotiator as NegotiatorBase,
-                        negotiate as _negotiate)
+from .negotiation import (Formatter as FormatterBase,
+                          Negotiator as NegotiatorBase,
+                          negotiate as _negotiate)
 
 
 logger = logging.getLogger(__name__)
diff --git a/swh/core/api/negotiate.py b/swh/core/api/negotiation.py
rename from swh/core/api/negotiate.py
rename to swh/core/api/negotiation.py
diff --git a/swh/core/config.py b/swh/core/config.py
--- a/swh/core/config.py
+++ b/swh/core/config.py
@@ -193,39 +193,40 @@
     - dict + val -> TypeError
     - val + val -> val (other)
 
-    so merging
+    for instance:
 
-    {
-      'key1': {
-        'skey1': value1,
-        'skey2': {'sskey1': value2},
-      },
-      'key2': value3,
-    }
+    >>> d1 = {
+    ...   'key1': {
+    ...     'skey1': 'value1',
+    ...     'skey2': {'sskey1': 'value2'},
+    ...   },
+    ...   'key2': 'value3',
+    ... }
 
     with
 
-    {
-      'key1': {
-        'skey1': value4,
-        'skey2': {'sskey2': value5},
-      },
-      'key3': value6,
-    }
+    >>> d2 = {
+    ...   'key1': {
+    ...     'skey1': 'value4',
+    ...     'skey2': {'sskey2': 'value5'},
+    ...   },
+    ...   'key3': 'value6',
+    ... }
 
     will give:
 
-    {
-      'key1': {
-        'skey1': value4,  # <-- note this
-        'skey2': {
-          'sskey1': value2,
-          'sskey2': value5,
-        },
-      },
-      'key2': value3,
-      'key3': value6,
-    }
+    >>> d3 = {
+    ...   'key1': {
+    ...     'skey1': 'value4',  # <-- note this
+    ...     'skey2': {
+    ...       'sskey1': 'value2',
+    ...       'sskey2': 'value5',
+    ...     },
+    ...   },
+    ...   'key2': 'value3',
+    ...   'key3': 'value6',
+    ... }
+    >>> assert merge_configs(d1, d2) == d3
 
     Note that no type checking is done for anything but dicts.
     """
diff --git a/tox.ini b/tox.ini
--- a/tox.ini
+++ b/tox.ini
@@ -7,7 +7,7 @@
   pytest-cov
   pifpaf
 commands =
-  pifpaf run postgresql -- pytest --hypothesis-profile=fast --cov=swh --cov-branch {posargs}
+  pifpaf run postgresql -- pytest --doctest-modules --hypothesis-profile=fast --cov=swh --cov-branch {posargs}
 
 [testenv:py3-slow]
 deps =
@@ -15,7 +15,7 @@
   pytest-cov
   pifpaf
 commands =
-  pifpaf run postgresql -- pytest --hypothesis-profile=slow --cov=swh --cov-branch {posargs}
+  pifpaf run postgresql -- pytest --doctest-modules --hypothesis-profile=slow --cov=swh --cov-branch {posargs}
 
 
 [testenv:flake8]