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, 7 insertions, 9 deletions

diff --git a/mesonbuild/build.py b/mesonbuild/build.py
index a5ebc34..c8d692e 100644
--- a/mesonbuild/build.py
+++ b/mesonbuild/build.py
@@ -115,7 +115,7 @@ class Build:
             self.compilers[lang] = compiler
 
     def add_cross_compiler(self, compiler):
-        if len(self.cross_compilers) == 0:
+        if not self.cross_compilers:
             self.static_cross_linker = self.environment.detect_static_linker(compiler)
         lang = compiler.get_language()
         if lang not in self.cross_compilers:
@@ -340,9 +340,7 @@ class BuildTarget(Target):
         self.process_objectlist(objects)
         self.process_kwargs(kwargs, environment)
         self.check_unknown_kwargs(kwargs)
-        if len(self.sources) == 0 \
-                and len(self.generated) == 0 \
-                and len(self.objects) == 0:
+        if not self.sources and not self.generated and not self.objects:
             raise InvalidArguments('Build target %s has no sources.' % name)
         self.process_compilers()
         self.validate_sources()
@@ -433,7 +431,7 @@ class BuildTarget(Target):
         We also add compilers that were used by extracted objects to simplify
         dynamic linker determination.
         '''
-        if len(self.sources) + len(self.generated) + len(self.objects) == 0:
+        if not self.sources and not self.generated and not self.objects:
             return
         # Populate list of compilers
         if self.is_cross:
@@ -488,7 +486,7 @@ class BuildTarget(Target):
             self.compilers['c'] = compilers['c']
 
     def validate_sources(self):
-        if len(self.sources) == 0:
+        if not self.sources:
             return
         for lang in ('cs', 'java'):
             if lang in self.compilers:
@@ -675,7 +673,7 @@ class BuildTarget(Target):
         if 'name_prefix' in kwargs:
             name_prefix = kwargs['name_prefix']
             if isinstance(name_prefix, list):
-                if len(name_prefix) != 0:
+                if name_prefix:
                     raise InvalidArguments('name_prefix array must be empty to signify null.')
             elif not isinstance(name_prefix, str):
                 raise InvalidArguments('name_prefix must be a string.')
@@ -684,7 +682,7 @@ class BuildTarget(Target):
         if 'name_suffix' in kwargs:
             name_suffix = kwargs['name_suffix']
             if isinstance(name_suffix, list):
-                if len(name_suffix) != 0:
+                if name_suffix:
                     raise InvalidArguments('name_suffix array must be empty to signify null.')
             else:
                 if not isinstance(name_suffix, str):
@@ -825,7 +823,7 @@ You probably should put it in link_with instead.''')
             self.link_whole_targets.append(t)
 
     def add_pch(self, language, pchlist):
-        if len(pchlist) == 0:
+        if not pchlist:
             return
         elif len(pchlist) == 1:
             if not environment.is_header(pchlist[0]):