allow fmt argument of conjoin to be a function.

doc/releases.rst

@@ -7,6 +7,7 @@ Releases
 
     - Allow :class:`inform.ProgressBar` output to be suppressed.
     - Allow ``/`` to be overridden in :class:`inform.plural`
+    - Various enhancements to :func:`inform.conjoin` and :func:`inform.full_stop`.
 
 **1.20 (2020-01-08)**:
     - Add *format* method to :class:`inform.plural`.

inform/inform.py

@@ -771,7 +771,9 @@ def conjoin(iterable, conj=' and ', sep=', ', end='', fmt=None):
             Is added to the end of the returned string.
         fmt (string):
             A format string used to convert each item in *iterable* to a string.
-            If not given, str() is used.
+            May be a function, in which case it called on each member of
+            *iterable* and must return a string.
+            If *fmt* is not given, str() is used.
 
     Return the items of the *iterable* joined into a string, where *conj* is
     used to join the last two items in the list, and *sep* is used to join the
@@ -831,9 +833,18 @@ def conjoin(iterable, conj=' and ', sep=', ', end='', fmt=None):
           carol : <carol@btca.com>; &
           alice : <alice@btca.com>.
 
+        >>> display(conjoin(characters, fmt=lambda a: f'{a.name:>7} : <{a.email}>', conj='\n', sep='\n'))
+            bob : <bob@btca.com>
+            ted : <ted@btca.com>
+          carol : <carol@btca.com>
+          alice : <alice@btca.com>
+
     """
     if fmt:
-        lst = [fmt.format(m) for m in iterable]
+        if callable(fmt):
+            lst = [fmt(m) for m in iterable]
+        else:
+            lst = [fmt.format(m) for m in iterable]
     else:
         lst = [str(m) for m in iterable]
     if conj and len(lst) > 1:

tests/test_utilities.py

@@ -172,7 +172,12 @@ def test_conjoin():
         alice : <alice@btca.com>.
     ''').strip()
 
-
+    assert conjoin(characters, fmt=lambda a: a.render('{name} : <{email}>'), conj='\n', sep='\n') == dedent('''
+        bob : <bob@btca.com>
+        ted : <ted@btca.com>
+        carol : <carol@btca.com>
+        alice : <alice@btca.com>
+    ''').strip()
 
 def test_did_you_mean():
     assert did_you_mean('abc', ['bcd']) == 'bcd'