[PATCH] trace: avoid "is" with a literal Python 3.8 warnings

[Stefan Hajnoczi]

The following statement produces a SyntaxWarning with Python 3.8:

  if len(format) is 0:
  scripts/tracetool/__init__.py:459: SyntaxWarning: "is" with a literal. Did 
you mean "=="?

Use the conventional len(x) == 0 syntax instead.

Reported-by: Daniel P. Berrangé <berra...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
 scripts/tracetool/__init__.py | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/scripts/tracetool/__init__.py b/scripts/tracetool/__init__.py
index 04279fa62e..44c118bc2a 100644
--- a/scripts/tracetool/__init__.py
+++ b/scripts/tracetool/__init__.py
@@ -456,12 +456,12 @@ def generate(events, group, format, backends,
     import tracetool
 
     format = str(format)
-    if len(format) is 0:
+    if len(format) == 0:
         raise TracetoolError("format not set")
     if not tracetool.format.exists(format):
         raise TracetoolError("unknown format: %s" % format)
 
-    if len(backends) is 0:
+    if len(backends) == 0:
         raise TracetoolError("no backends specified")
     for backend in backends:
         if not tracetool.backend.exists(backend):
-- 
2.21.0