Don't use len() to test emptiness vs not emptiness

Meson has a common pattern of using 'if len(foo) == 0:' or
'if len(foo) != 0:', however, this is a common anti-pattern in python.
Instead tests for emptiness/non-emptiness should be done with a simple
'if foo:' or 'if not foo:'

Consider the following:
>>> import timeit
>>> timeit.timeit('if len([]) == 0: pass')
0.10730923599840025
>>> timeit.timeit('if not []: pass')
0.030033907998586074
>>> timeit.timeit('if len(['a', 'b', 'c', 'd']) == 0: pass')
0.1154778649979562
>>> timeit.timeit("if not ['a', 'b', 'c', 'd']: pass")
0.08259823200205574
>>> timeit.timeit('if len("") == 0: pass')
0.089759664999292
>>> timeit.timeit('if not "": pass')
0.02340641999762738
>>> timeit.timeit('if len("foo") == 0: pass')
0.08848102600313723
>>> timeit.timeit('if not "foo": pass')
0.04032287199879647

And for the one additional case of 'if len(foo.strip()) == 0', which can
be replaced with 'if not foo.isspace()'
>>> timeit.timeit('if len("   ".strip()) == 0: pass')
0.15294511600222904
>>> timeit.timeit('if "   ".isspace(): pass')
0.09413968399894657
>>> timeit.timeit('if len("   abc".strip()) == 0: pass')
0.2023209120015963
>>> timeit.timeit('if "   abc".isspace(): pass')
0.09571301700270851

In other words, it's always a win to not use len(), when you don't
actually want to check the length.

1 files changed, 2 insertions, 2 deletions

diff --git a/mesonbuild/mesonmain.py b/mesonbuild/mesonmain.py
index 7e4be7f..5544037 100644
--- a/mesonbuild/mesonmain.py
+++ b/mesonbuild/mesonmain.py
@@ -269,10 +269,10 @@ def run(mainfile, args):
     args = mesonlib.expand_arguments(args)
     options = parser.parse_args(args)
     args = options.directories
-    if len(args) == 0 or len(args) > 2:
+    if not args or len(args) > 2:
         # if there's a meson.build in the dir above, and not in the current
         # directory, assume we're in the build directory
-        if len(args) == 0 and not os.path.exists('meson.build') and os.path.exists('../meson.build'):
+        if not args and not os.path.exists('meson.build') and os.path.exists('../meson.build'):
             dir1 = '..'
             dir2 = '.'
         else: