capture_output does not respect trailing semicolon (#13940)

This change tests whether the Python code inside the `%%capture` magic
ends with a `';'`. If it does, the code no longer creates the object
that is supposed to store the cell output. If such object already
exists, it is deleted.
Please review and suggest changes if you think they can improve the
solution. Thanks.

IPython/core/magics/execution.py

@@ -47,6 +47,7 b' from IPython.utils.ipstruct import Struct'
 from IPython.utils.module_paths import find_mod
 from IPython.utils.path import get_py_filename, shellglob
 from IPython.utils.timing import clock, clock2
+from IPython.core.displayhook import DisplayHook
 
 #-----------------------------------------------------------------------------
 # Magic implementation classes
@@ -1461,7 +1462,10 b' class ExecutionMagics(Magics):'
         disp = not args.no_display
         with capture_output(out, err, disp) as io:
             self.shell.run_cell(cell)
-        if args.output:
+        if DisplayHook.semicolon_at_end_of_expression(cell):
+            if args.output in self.shell.user_ns:
+                del self.shell.user_ns[args.output]
+        elif args.output:
             self.shell.user_ns[args.output] = io
 
 def parse_breakpoint(text, current_file):

IPython/core/tests/test_magic.py

@@ -530,6 +530,25 b' def test_time_local_ns():'
     del ip.user_ns["myvar"]
 
 
+# Test %%capture magic. Added to test issue #13926
+def test_capture():
+    ip = get_ipython()
+
+    # Test %%capture nominal case
+    ip.run_cell("%%capture abc\n1+2")
+    with tt.AssertPrints("True", suppress=False):
+        ip.run_cell("'abc' in locals()")
+    with tt.AssertPrints("True", suppress=False):
+        ip.run_cell("'outputs' in dir(abc)")
+    with tt.AssertPrints("3", suppress=False):
+        ip.run_cell("abc.outputs[0]")
+
+    # Test %%capture with ';' at end of expression
+    ip.run_cell("%%capture abc\n7+8;")
+    with tt.AssertPrints("False", suppress=False):
+        ip.run_cell("'abc' in locals()")
+
+
 def test_doctest_mode():
     "Toggle doctest_mode twice, it should be a no-op and run without error"
     _ip.run_line_magic("doctest_mode", "")