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, 6 insertions, 6 deletions

diff --git a/mesonbuild/mconf.py b/mesonbuild/mconf.py
index 2ab5f92..b4add65 100644
--- a/mesonbuild/mconf.py
+++ b/mesonbuild/mconf.py
@@ -56,7 +56,7 @@ class Conf:
         # Ninja is run.
 
     def print_aligned(self, arr):
-        if len(arr) == 0:
+        if not arr:
             return
         titles = ['Option', 'Description', 'Current Value', '']
         longest_name = len(titles[0])
@@ -139,7 +139,7 @@ class Conf:
         print('')
         print('Base options:')
         okeys = sorted(self.coredata.base_options.keys())
-        if len(okeys) == 0:
+        if not okeys:
             print('  No base options\n')
         else:
             coarr = []
@@ -158,7 +158,7 @@ class Conf:
         print('')
         print('Compiler options:')
         okeys = sorted(self.coredata.compiler_options.keys())
-        if len(okeys) == 0:
+        if not okeys:
             print('  No compiler options\n')
         else:
             coarr = []
@@ -188,7 +188,7 @@ class Conf:
         self.print_aligned(parr)
         print('')
         print('Project options:')
-        if len(self.coredata.user_options) == 0:
+        if not self.coredata.user_options:
             print('  This project does not have any options')
         else:
             options = self.coredata.user_options
@@ -197,7 +197,7 @@ class Conf:
             optarr = []
             for key in keys:
                 opt = options[key]
-                if (opt.choices is None) or (len(opt.choices) == 0):
+                if (opt.choices is None) or (not opt.choices):
                     # Zero length list or string
                     choices = ''
                 else:
@@ -222,7 +222,7 @@ def run(args):
         print('%s <build directory>' % args[0])
         print('If you omit the build directory, the current directory is substituted.')
         return 1
-    if len(options.directory) == 0:
+    if not options.directory:
         builddir = os.getcwd()
     else:
         builddir = options.directory[0]