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, 8 insertions, 8 deletions

diff --git a/mesonbuild/compilers.py b/mesonbuild/compilers.py
index 188bd8f..ba0fb4e 100644
--- a/mesonbuild/compilers.py
+++ b/mesonbuild/compilers.py
@@ -317,12 +317,12 @@ def get_base_link_args(options, linker, is_shared_module):
     return args
 
 def build_unix_rpath_args(build_dir, rpath_paths, install_rpath):
-        if len(rpath_paths) == 0 and len(install_rpath) == 0:
+        if not rpath_paths and not install_rpath:
             return []
         paths = ':'.join([os.path.join(build_dir, p) for p in rpath_paths])
         if len(paths) < len(install_rpath):
             padding = 'X' * (len(install_rpath) - len(paths))
-            if len(paths) == 0:
+            if not paths:
                 paths = padding
             else:
                 paths = paths + ':' + padding
@@ -388,7 +388,7 @@ class CompilerArgs(list):
         if len(args) > 2:
             raise TypeError("CompilerArgs() only accepts at most 2 arguments: "
                             "The compiler, and optionally an initial list")
-        elif len(args) == 0:
+        elif not args:
             return cargs
         elif len(args) == 1:
             if isinstance(args[0], (Compiler, StaticLinker)):
@@ -708,7 +708,7 @@ class Compiler:
         return self.get_std_shared_lib_link_args()
 
     def get_link_whole_for(self, args):
-        if isinstance(args, list) and len(args) == 0:
+        if isinstance(args, list) and not args:
             return []
         raise EnvironmentException('Language %s does not support linking whole archives.' % self.language)
 
@@ -1360,7 +1360,7 @@ class CCompiler(Compiler):
             extra_dirs = [extra_dirs]
         # Gcc + co seem to prefer builtin lib dirs to -L dirs.
         # Only try to find std libs if no extra dirs specified.
-        if len(extra_dirs) == 0:
+        if not extra_dirs:
             args = ['-l' + libname]
             if self.links(code, env, extra_args=args):
                 return args
@@ -1700,7 +1700,7 @@ class ValaCompiler(Compiler):
             extra_dirs = [extra_dirs]
         # Valac always looks in the default vapi dir, so only search there if
         # no extra dirs are specified.
-        if len(extra_dirs) == 0:
+        if not extra_dirs:
             code = 'class MesonFindLibrary : Object { }'
             vapi_args = ['--pkg', libname]
             args = self.get_cross_extra_flags(env, link=False)
@@ -1892,12 +1892,12 @@ class DCompiler(Compiler):
     def build_rpath_args(self, build_dir, rpath_paths, install_rpath):
         # This method is to be used by LDC and DMD.
         # GDC can deal with the verbatim flags.
-        if len(rpath_paths) == 0 and len(install_rpath) == 0:
+        if not rpath_paths and not install_rpath:
             return []
         paths = ':'.join([os.path.join(build_dir, p) for p in rpath_paths])
         if len(paths) < len(install_rpath):
             padding = 'X' * (len(install_rpath) - len(paths))
-            if len(paths) == 0:
+            if not paths:
                 paths = padding
             else:
                 paths = paths + ':' + padding