visit_Assert: minor cleanup

- import format_explanation as-is
- move setting of `negation` out of branches

src/_pytest/assertion/rewrite.py

@@ -22,9 +22,7 @@
 from _pytest._io.saferepr import saferepr
 from _pytest._version import version
 from _pytest.assertion import util
-from _pytest.assertion.util import (  # noqa: F401
-    format_explanation as _format_explanation,
-)
+from _pytest.assertion.util import format_explanation  # noqa: F401
 from _pytest.compat import fspath
 from _pytest.pathlib import fnmatch_ex
 from _pytest.pathlib import Path
@@ -807,8 +805,9 @@ def visit_Assert(self, assert_):
                 )
             )
 
+        negation = ast.UnaryOp(ast.Not(), top_condition)
+
         if self.enable_assertion_pass_hook:  # Experimental pytest_assertion_pass hook
-            negation = ast.UnaryOp(ast.Not(), top_condition)
             msg = self.pop_format_context(ast.Str(explanation))
 
             # Failed
@@ -821,15 +820,15 @@ def visit_Assert(self, assert_):
             err_explanation = ast.BinOp(ast.Str(gluestr), ast.Add(), msg)
             err_msg = ast.BinOp(assertmsg, ast.Add(), err_explanation)
             err_name = ast.Name("AssertionError", ast.Load())
-            fmt = self.helper("_format_explanation", err_msg)
+            fmt = self.helper("format_explanation", err_msg)
             exc = ast.Call(err_name, [fmt], [])
             raise_ = ast.Raise(exc, None)
             statements_fail = []
             statements_fail.extend(self.expl_stmts)
             statements_fail.append(raise_)
 
             # Passed
-            fmt_pass = self.helper("_format_explanation", msg)
+            fmt_pass = self.helper("format_explanation", msg)
             orig = self._assert_expr_to_lineno()[assert_.lineno]
             hook_call_pass = ast.Expr(
                 self.helper(
@@ -860,7 +859,6 @@ def visit_Assert(self, assert_):
         else:  # Original assertion rewriting
             # Create failure message.
             body = self.expl_stmts
-            negation = ast.UnaryOp(ast.Not(), top_condition)
             self.statements.append(ast.If(negation, body, []))
             if assert_.msg:
                 assertmsg = self.helper("_format_assertmsg", assert_.msg)
@@ -870,7 +868,7 @@ def visit_Assert(self, assert_):
                 explanation = "assert " + explanation
             template = ast.BinOp(assertmsg, ast.Add(), ast.Str(explanation))
             msg = self.pop_format_context(template)
-            fmt = self.helper("_format_explanation", msg)
+            fmt = self.helper("format_explanation", msg)
             err_name = ast.Name("AssertionError", ast.Load())
             exc = ast.Call(err_name, [fmt], [])
             raise_ = ast.Raise(exc, None)