Add separator to FieldList

CHANGES.rst

@@ -14,6 +14,7 @@ Unreleased
 -   Support for optgroups in :class:`~fields.SelectField` and
     :class:`~fields.SelectMultipleField`. :issue:`656` :pr:`667`
 -   Minor documentation fix. :issue:`701`
+-   Custom separators for :class:`~fields.FieldList`. :issue:`681` :pr:`694`
 
 Version 3.0.0a1
 ---------------

docs/fields.rst

@@ -414,7 +414,7 @@ complex data structures such as lists and nested objects can be represented.
     :attr:`~wtforms.form.Form.data` dict of the enclosed form. Similarly, the
     `errors` property encapsulate the forms' errors.
 
-.. autoclass:: FieldList(unbound_field, default field arguments, min_entries=0, max_entries=None)
+.. autoclass:: FieldList(unbound_field, default field arguments, min_entries=0, max_entries=None, separator='-')
 
     **Note**: Due to a limitation in how HTML sends values, FieldList cannot enclose
     :class:`BooleanField` or :class:`SubmitField` instances.

src/wtforms/fields/core.py

@@ -1105,6 +1105,9 @@ class FieldList(Field):
     :param max_entries:
         accept no more than this many entries as input, even if more exist in
         formdata.
+    :param separator:
+        A string which will be suffixed to this field's name to create the
+        prefix to enclosed list entries. The default is fine for most uses.
     """
 
     widget = widgets.ListWidget()
@@ -1116,6 +1119,7 @@ def __init__(
         validators=None,
         min_entries=0,
         max_entries=None,
+        separator="-",
         default=(),
         **kwargs,
     ):
@@ -1133,6 +1137,8 @@ def __init__(
         self.max_entries = max_entries
         self.last_index = -1
         self._prefix = kwargs.get("_prefix", "")
+        self._separator = separator
+        self._field_separator = unbound_field.kwargs.get("separator", "-")
 
     def process(self, formdata, data=unset_value, extra_filters=None):
         if extra_filters:
@@ -1175,12 +1181,12 @@ def _extract_indices(self, prefix, formdata):
 
         formdata must be an object which will produce keys when iterated.  For
         example, if field 'foo' contains keys 'foo-0-bar', 'foo-1-baz', then
-        the numbers 0 and 1 will be yielded, but not neccesarily in order.
+        the numbers 0 and 1 will be yielded, but not necessarily in order.
         """
         offset = len(prefix) + 1
         for k in formdata:
             if k.startswith(prefix):
-                k = k[offset:].split("-", 1)[0]
+                k = k[offset:].split(self._field_separator, 1)[0]
                 if k.isdigit():
                     yield int(k)
 
@@ -1232,8 +1238,8 @@ def _add_entry(self, formdata=None, data=unset_value, index=None):
         if index is None:
             index = self.last_index + 1
         self.last_index = index
-        name = "%s-%d" % (self.short_name, index)
-        id = "%s-%d" % (self.id, index)
+        name = f"{self.short_name}{self._separator}{index}"
+        id = f"{self.id}{self._separator}{index}"
         field = self.unbound_field.bind(
             form=None,
             name=name,

tests/fields/test_list.py

@@ -103,6 +103,68 @@ class Outside(Form):
     assert o.subforms[0].foo.data == "default"
 
 
+def test_custom_separator():
+    F = make_form(a=FieldList(t, separator="_"))
+
+    pdata = DummyPostData({"a_0": "0_a", "a_1": "1_a"})
+    f = F(pdata)
+    assert f.a[0].data == "0_a"
+    assert f.a[1].data == "1_a"
+
+
+def test_enclosed_subform_list_separator():
+    class Inside(Form):
+        foo = StringField(default="default")
+
+    class Outside(Form):
+        subforms = FieldList(FormField(Inside), min_entries=1, separator="_")
+
+    o = Outside()
+    assert o.subforms[0].foo.data == "default"
+    assert o.subforms[0].foo.name == "subforms_0-foo"
+
+    pdata = DummyPostData({"subforms_0-foo": "0-foo", "subforms_1-foo": "1-foo"})
+    o = Outside(pdata)
+    assert o.subforms[0].foo.data == "0-foo"
+    assert o.subforms[1].foo.data == "1-foo"
+
+
+def test_enclosed_subform_uniform_separators():
+    class Inside(Form):
+        foo = StringField(default="default")
+
+    class Outside(Form):
+        subforms = FieldList(
+            FormField(Inside, separator="_"), min_entries=1, separator="_"
+        )
+
+    o = Outside()
+    assert o.subforms[0].foo.data == "default"
+    assert o.subforms[0].foo.name == "subforms_0_foo"
+
+    pdata = DummyPostData({"subforms_0_foo": "0_foo", "subforms_1_foo": "1_foo"})
+    o = Outside(pdata)
+    assert o.subforms[0].foo.data == "0_foo"
+    assert o.subforms[1].foo.data == "1_foo"
+
+
+def test_enclosed_subform_mixed_separators():
+    class Inside(Form):
+        foo = StringField(default="default")
+
+    class Outside(Form):
+        subforms = FieldList(FormField(Inside, separator="_"), min_entries=1)
+
+    o = Outside()
+    assert o.subforms[0].foo.data == "default"
+    assert o.subforms[0].foo.name == "subforms-0_foo"
+
+    pdata = DummyPostData({"subforms-0_foo": "0_foo", "subforms-1_foo": "1_foo"})
+    o = Outside(pdata)
+    assert o.subforms[0].foo.data == "0_foo"
+    assert o.subforms[1].foo.data == "1_foo"
+
+
 def test_entry_management():
     F = make_form(a=FieldList(t))
     a = F(a=["hello", "bye"]).a