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, 7 deletions

diff --git a/mesonbuild/interpreterbase.py b/mesonbuild/interpreterbase.py
index e59557a..86a6b47 100644
--- a/mesonbuild/interpreterbase.py
+++ b/mesonbuild/interpreterbase.py
@@ -34,7 +34,7 @@ def check_stringlist(a, msg='Arguments must be strings.'):
 def noPosargs(f):
     @wraps(f)
     def wrapped(self, node, args, kwargs):
-        if len(args) != 0:
+        if args:
             raise InvalidArguments('Function does not take positional arguments.')
         return f(self, node, args, kwargs)
     return wrapped
@@ -42,7 +42,7 @@ def noPosargs(f):
 def noKwargs(f):
     @wraps(f)
     def wrapped(self, node, args, kwargs):
-        if len(kwargs) != 0:
+        if kwargs:
             raise InvalidArguments('Function does not take keyword arguments.')
         return f(self, node, args, kwargs)
     return wrapped
@@ -94,7 +94,7 @@ class InterpreterBase:
             raise InvalidArguments('Missing Meson file in %s' % mesonfile)
         with open(mesonfile, encoding='utf8') as mf:
             code = mf.read()
-        if len(code.strip()) == 0:
+        if code.isspace():
             raise InvalidCode('Builder file is empty.')
         assert(isinstance(code, str))
         try:
@@ -113,7 +113,7 @@ class InterpreterBase:
     def sanity_check_ast(self):
         if not isinstance(self.ast, mparser.CodeBlockNode):
             raise InvalidCode('AST is of invalid type. Possibly a bug in the parser.')
-        if len(self.ast.lines) == 0:
+        if not self.ast.lines:
             raise InvalidCode('No statements in code.')
         first = self.ast.lines[0]
         if not isinstance(first, mparser.FunctionNode) or first.func_name != 'project':
@@ -405,7 +405,7 @@ class InterpreterBase:
         obj = self.to_native(obj)
         (posargs, _) = self.reduce_arguments(args)
         if method_name == 'to_string':
-            if len(posargs) == 0:
+            if not posargs:
                 if obj:
                     return 'true'
                 else:
@@ -429,12 +429,12 @@ class InterpreterBase:
         obj = self.to_native(obj)
         (posargs, _) = self.reduce_arguments(args)
         if method_name == 'is_even':
-            if len(posargs) == 0:
+            if not posargs:
                 return obj % 2 == 0
             else:
                 raise InterpreterException('int.is_even() must have no arguments.')
         elif method_name == 'is_odd':
-            if len(posargs) == 0:
+            if not posargs:
                 return obj % 2 != 0
             else:
                 raise InterpreterException('int.is_odd() must have no arguments.')