The trace module command-line utility doesn't write cover files. I've noticed this issue for some years now. It works fine in Python 2. When using Python 3, no ".cover" files are written, regardless of how "--coverdir" is specified.

mike on macbook in ~/
$ echo 'print("hello")' > foo.py

mike on macbook in ~/
$ python -m trace --count foo.py
hello

mike on macbook in ~/
$ ls *.cover
ls: *.cover: No such file or directory

My apologies if this is a duplicate bug. I searched the tracker and Google for a while, but couldn't find a relevant issue.

It worked in Python 3.4, but not afterwards.

The problem appears to be a mistake in commit f026dae

f026dae

This made the writing of cover files conditional on show_missing which is the option to mark lines which weren't executed with several angle brackets ">>>>>".

https://github.com/python/cpython/blob/3.5/Lib/trace.py#L326

mike on mac in ~/
$ python -m trace --count foo.py
hello

mike on mac in ~/
$ ls *.cover
ls: *.cover: No such file or directory

mike on mac in ~/
$ python -m trace --count -m foo.py
hello

mike on mac in ~/
$ ls *.cover
-rw-r--r--  1 mike  staff    22B Oct 31 15:40 foo.cover

While writing a patch for this, I noticed the lnotab parameter seems nearly unused. It's a dict, but is only used for its keys.

https://github.com/python/cpython/blob/master/Lib/trace.py#L333

Further, the choice to count unreached lines only when show_missing was set seems inconsistent.

https://github.com/python/cpython/blob/master/Lib/trace.py#L335

https://github.com/python/cpython/blob/master/Lib/trace.py#L280

Ok, pull request submitted:
#4205

I think the first part of your patch also fixes bpo-26818. Could you adapt the test there and add a test case for the problem in this issue?

You're referring to something like this:

+ def test_count_and_summary(self):
+ name = TESTFN + '.py'
+ with open(name, 'w') as fd:
+ self.addCleanup(unlink, name)
+ fd.write("""\
+x = 1
+y = 2
+
+def f():
+ return x + y
+
+for i in range(10):
+ f()
+ """)
+ status, stdout, stderr = assert_python_ok('-m', 'trace', '-cs', name)
+ self.assertEqual(status, 0)
+ self.assertIn(b'lines cov% module (path)', stdout)
+ self.assertIn(('6 100%% %s (%s)' % (TESTFN, name)).encode(), stdout)
+

?

New changeset 47ab154 by Serhiy Storchaka (Michael Selik) in branch 'master':
bpo-31908: Fix output of cover files for trace module command-line tool. (GH-4205)
47ab154

New changeset e4eeb6e by Miss Islington (bot) in branch '3.7':
bpo-31908: Fix output of cover files for trace module command-line tool. (GH-4205)
e4eeb6e

New changeset a607f8b by Miss Islington (bot) in branch '3.6':
bpo-31908: Fix output of cover files for trace module command-line tool. (GH-4205)
a607f8b

This change causes test_trace to leave a trace.cover file in the Lib directory (see bpo-34171).

New changeset c8b0dbc by Berker Peksag in branch 'master':
bpo-26818: Add a test to make sure the bug is fixed (GH-8664)
c8b0dbc

New changeset 8fc21c8 by Berker Peksag (Miss Islington (bot)) in branch '3.7':
bpo-26818: Add a test to make sure the bug is fixed (GH-8664)
8fc21c8