Fix signature rendering

IPython/core/oinspect.py

@@ -1051,7 +1051,9 b' def _render_signature(obj_signature, obj_name):'
     # add up name, parameters, braces (2), and commas
     if len(obj_name) + sum(len(r) + 2 for r in result) > 75:
         # This doesn’t fit behind “Signature: ” in an inspect window.
-        rendered = '{}(\n{})'.format(obj_name, ''.join('    {},\n'.format(result)))
+        rendered = '{}(\n{})'.format(obj_name, ''.join(
+            '    {},\n'.format(r) for r in result)
+        )
     else:
         rendered = '{}({})'.format(obj_name, ', '.join(result))
 

IPython/core/tests/test_oinspect.py

@@ -5,7 +5,7 b''
 # Distributed under the terms of the Modified BSD License.
 
 
-from inspect import Signature, Parameter
+from inspect import signature, Signature, Parameter
 import os
 import re
 import sys
@@ -22,7 +22,6 b' from IPython import get_ipython'
 from IPython.testing.tools import AssertPrints, AssertNotPrints
 from IPython.utils.path import compress_user
 
-
 #-----------------------------------------------------------------------------
 # Globals and constants
 #-----------------------------------------------------------------------------
@@ -432,3 +431,33 b' def test_builtin_init():'
     init_def = info['init_definition']
     nt.assert_is_not_none(init_def)
 
+
+def test_render_signature_short():
+    def short_fun(a: int = 1): pass
+    sig = oinspect._render_signature(
+        signature(short_fun),
+        short_fun.__name__,
+    )
+    nt.assert_equal(sig, 'short_fun(a: int = 1)')
+
+
+def test_render_signature_long():
+    from typing import Optional
+
+    def long_function(
+        a_really_long_parameter: int,
+        and_another_long_one: bool = False,
+        let_us_make_sure_this_is_looong: Optional[str] = None,
+    ) -> bool: pass
+
+    sig = oinspect._render_signature(
+        signature(long_function),
+        long_function.__name__,
+    )
+    nt.assert_equal(sig, '''\
+long_function(
+    a_really_long_parameter: int,
+    and_another_long_one: bool = False,
+    let_us_make_sure_this_is_looong: Union[str, NoneType] = None,
+) -> bool\
+''')