Don't expand user variables in execution magics

Adds magic.no_var_expand decorator to mark a magic as opting out of variable expansion

Most useful for magics that take Python code on the line, e.g. %timeit, etc.

IPython/core/interactiveshell.py

@@ -2276,6 +2276,10 b' class InteractiveShell(SingletonConfigurable):'
             
             # Determine stack_depth depending on where run_line_magic() has been called
             stack_depth = _stack_depth
+            if getattr(fn, magic.MAGIC_NO_VAR_EXPAND_ATTR, False):
+                # magic has opted out of var_expand
+                magic_arg_s = line
+            else:
             magic_arg_s = self.var_expand(line, stack_depth)
             # Put magic args in a list so we can call with f(*a) syntax
             args = [magic_arg_s]
@@ -2318,6 +2322,10 b' class InteractiveShell(SingletonConfigurable):'
             # This will need to be updated if the internal calling logic gets
             # refactored, or else we'll be expanding the wrong variables.
             stack_depth = 2
+            if getattr(fn, magic.MAGIC_NO_VAR_EXPAND_ATTR, False):
+                # magic has opted out of var_expand
+                magic_arg_s = line
+            else:
             magic_arg_s = self.var_expand(line, stack_depth)
             with self.builtin_trap:
                 result = fn(magic_arg_s, cell)

IPython/core/magic.py

@@ -265,6 +265,25 b' def _function_magic_marker(magic_kind):'
     return magic_deco
 
 
+MAGIC_NO_VAR_EXPAND_ATTR = '_ipython_magic_no_var_expand'
+
+
+def no_var_expand(magic_func):
+    """Mark a magic function as not needing variable expansion
+
+    By default, IPython interprets `{a}` or `$a` in the line passed to magics
+    as variables that should be interpolated from the interactive namespace
+    before passing the line to the magic function.
+    This is not always desirable, e.g. when the magic executes Python code
+    (%timeit, %time, etc.).
+    Decorate magics with `@no_var_expand` to opt-out of variable expansion.
+
+    .. versionadded:: 7.2
+    """
+    setattr(magic_func, MAGIC_NO_VAR_EXPAND_ATTR, True)
+    return magic_func
+
+
 # Create the actual decorators for public use
 
 # These three are used to decorate methods in class definitions

IPython/core/magics/execution.py

@@ -36,7 +36,8 b' from IPython.core import page'
 from IPython.core.error import UsageError
 from IPython.core.macro import Macro
 from IPython.core.magic import (Magics, magics_class, line_magic, cell_magic,
-                                line_cell_magic, on_off, needs_local_scope)
+                                line_cell_magic, on_off, needs_local_scope,
+                                no_var_expand)
 from IPython.testing.skipdoctest import skip_doctest
 from IPython.utils.contexts import preserve_keys
 from IPython.utils.capture import capture_output
@@ -184,6 +185,7 b' python packages because of its non-free license. To use profiling, install the'
 python-profiler package from non-free.""")
 
     @skip_doctest
+    @no_var_expand
     @line_cell_magic
     def prun(self, parameter_s='', cell=None):
 
@@ -293,6 +295,11 b' python-profiler package from non-free.""")'
         You can read the complete documentation for the profile module with::
 
           In [1]: import profile; profile.help()
+
+        .. versionchanged:: 7.2
+            User variables are no longer expanded,
+            the magic line is always left unmodified.
+
         """
         opts, arg_str = self.parse_options(parameter_s, 'D:l:rs:T:q',
                                            list_all=True, posix=False)
@@ -422,6 +429,7 b' python-profiler package from non-free.""")'
         You can omit this in cell magic mode.
         """
     )
+    @no_var_expand
     @line_cell_magic
     def debug(self, line='', cell=None):
         """Activate the interactive debugger.
@@ -442,6 +450,11 b' python-profiler package from non-free.""")'
 
         If you want IPython to automatically do this on every exception, see
         the %pdb magic for more details.
+
+        .. versionchanged:: 7.2
+            When running code, user variables are no longer expanded,
+            the magic line is always left unmodified.
+
         """
         args = magic_arguments.parse_argstring(self.debug, line)
 
@@ -972,6 +985,7 b' python-profiler package from non-free.""")'
         print("Wall time: %10.2f s." % (twall1 - twall0))
 
     @skip_doctest
+    @no_var_expand
     @line_cell_magic
     @needs_local_scope
     def timeit(self, line='', cell=None, local_ns=None):
@@ -1017,6 +1031,9 b' python-profiler package from non-free.""")'
         -o: return a TimeitResult that can be stored in a variable to inspect
             the result in more details.
 
+        .. versionchanged:: 7.2
+            User variables are no longer expanded,
+            the magic line is always left unmodified.
 
         Examples
         --------
@@ -1161,6 +1178,7 b' python-profiler package from non-free.""")'
             return timeit_result
 
     @skip_doctest
+    @no_var_expand
     @needs_local_scope
     @line_cell_magic
     def time(self,line='', cell=None, local_ns=None):
@@ -1181,6 +1199,10 b' python-profiler package from non-free.""")'
         This function provides very basic timing functionality.  Use the timeit 
         magic for more control over the measurement.
 
+        .. versionchanged:: 7.2
+            User variables are no longer expanded,
+            the magic line is always left unmodified.
+
         Examples
         --------
         ::

IPython/core/tests/test_magic.py

@@ -1088,6 +1088,7 b' def test_logging_magic_quiet_from_config():'
         finally:
             _ip.logger.logstop()
     
+
 def test_logging_magic_not_quiet():
     _ip.config.LoggingMagics.quiet = False
     lm = logging.LoggingMagics(shell=_ip)
@@ -1098,9 +1099,15 b' def test_logging_magic_not_quiet():'
         finally:
             _ip.logger.logstop()
 
-## 
+
+def test_time_no_var_expand():
+    _ip.user_ns['a'] = 5
+    _ip.user_ns['b'] = []
+    _ip.magic('time b.append("{a}")')
+    assert _ip.user_ns['b'] == ['{a}']
+
+
 # this is slow, put at the end for local testing.
-## 
 def test_timeit_arguments():
     "Test valid timeit arguments, should not cause SyntaxError (GH #1269)"
     if sys.version_info < (3,7):