[pylint] Implementing len-as-condition (PLC1802)

crates/ruff_linter/resources/test/fixtures/pylint/len_as_condition.py

@@ -0,0 +1,231 @@
+if len('TEST'):  # [PLC1802]
+    pass
+
+if not len('TEST'):  # [PLC1802]
+    pass
+
+z = []
+if z and len(['T', 'E', 'S', 'T']):  # [PLC1802]
+    pass
+
+if True or len('TEST'):  # [PLC1802]
+    pass
+
+if len('TEST') == 0:  # Should be fine
+    pass
+
+if len('TEST') < 1:  # Should be fine
+    pass
+
+if len('TEST') <= 0:  # Should be fine
+    pass
+
+if 1 > len('TEST'):  # Should be fine
+    pass
+
+if 0 >= len('TEST'):  # Should be fine
+    pass
+
+if z and len('TEST') == 0:  # Should be fine
+    pass
+
+if 0 == len('TEST') < 10:  # Should be fine
+    pass
+
+# Should be fine
+if 0 < 1 <= len('TEST') < 10:  # [comparison-of-constants]
+    pass
+
+if 10 > len('TEST') != 0:  # Should be fine
+    pass
+
+if 10 > len('TEST') > 1 > 0:  # Should be fine
+    pass
+
+if 0 <= len('TEST') < 100:  # Should be fine
+    pass
+
+if z or 10 > len('TEST') != 0:  # Should be fine
+    pass
+
+if z:
+    pass
+elif len('TEST'):  # [PLC1802]
+    pass
+
+if z:
+    pass
+elif not len('TEST'):  # [PLC1802]
+    pass
+
+while len('TEST'):  # [PLC1802]
+    pass
+
+while not len('TEST'):  # [PLC1802]
+    pass
+
+while z and len('TEST'):  # [PLC1802]
+    pass
+
+while not len('TEST') and z:  # [PLC1802]
+    pass
+
+assert len('TEST') > 0  # Should be fine
+
+x = 1 if len('TEST') != 0 else 2  # Should be fine
+
+f_o_o = len('TEST') or 42  # Should be fine
+
+a = x and len(x)  # Should be fine
+
+def some_func():
+    return len('TEST') > 0  # Should be fine
+
+def github_issue_1325():
+    l = [1, 2, 3]
+    length = len(l) if l else 0  # Should be fine
+    return length
+
+def github_issue_1331(*args):
+    assert False, len(args)  # Should be fine
+
+def github_issue_1331_v2(*args):
+    assert len(args), args  # [PLC1802]
+
+def github_issue_1331_v3(*args):
+    assert len(args) or z, args  # [PLC1802]
+
+def github_issue_1331_v4(*args):
+    assert z and len(args), args  # [PLC1802]
+
+b = bool(len(z)) # [PLC1802]
+c = bool(len('TEST') or 42) # [PLC1802]
+
+def github_issue_1879():
+
+    class ClassWithBool(list):
+        def __bool__(self):
+            return True
+
+    class ClassWithoutBool(list):
+        pass
+
+    class ChildClassWithBool(ClassWithBool):
+        pass
+
+    class ChildClassWithoutBool(ClassWithoutBool):
+        pass
+
+    assert len(ClassWithBool())
+    assert len(ChildClassWithBool())
+    assert len(ClassWithoutBool())  # unintuitive?, in pylint: [PLC1802]
+    assert len(ChildClassWithoutBool())  # unintuitive?, in pylint: [PLC1802]
+    assert len(range(0))  # in pylint does throw [PLC1802] (stronger type inference), not here
+    assert len([t + 1 for t in []])  # [PLC1802]
+    assert len(u + 1 for u in [])  # [PLC1802]
+    assert len({"1":(v + 1) for v in {}})  # [PLC1802]
+    assert len(set((w + 1) for w in set()))  # [PLC1802]
+
+
+    import numpy
+    numpy_array = numpy.array([0])
+    if len(numpy_array) > 0:
+        print('numpy_array')
+    if len(numpy_array):
+        print('numpy_array')
+    if numpy_array:
+        print('b')
+
+    import pandas as pd
+    pandas_df = pd.DataFrame()
+    if len(pandas_df):
+        print("this works, but pylint tells me not to use len() without comparison")
+    if len(pandas_df) > 0:
+        print("this works and pylint likes it, but it's not the solution intended by PEP-8")
+    if pandas_df:
+        print("this does not work (truth value of dataframe is ambiguous)")
+
+    def function_returning_list(r):
+        if r==1:
+            return [1]
+        return [2]
+
+    def function_returning_int(r):
+        if r==1:
+            return 1
+        return 2
+
+    def function_returning_generator(r):
+        for i in [r, 1, 2, 3]:
+            yield i
+
+    def function_returning_comprehension(r):
+        return [x+1 for x in [r, 1, 2, 3]]
+
+    def function_returning_function(r):
+        return function_returning_generator(r)
+
+    assert len(function_returning_list(z))  # [PLC1802] differs from pylint 
+    assert len(function_returning_int(z))
+    # This should raise a PLC1802 once astroid can infer it
+    # See https://github.com/pylint-dev/pylint/pull/3821#issuecomment-743771514
+    assert len(function_returning_generator(z))
+    assert len(function_returning_comprehension(z))
+    assert len(function_returning_function(z))
+
+
+def github_issue_4215():
+    # Test undefined variables
+    # https://github.com/pylint-dev/pylint/issues/4215
+    if len(undefined_var):  # [undefined-variable]
+        pass
+    if len(undefined_var2[0]):  # [undefined-variable]
+        pass
+
+
+def f(cond:bool):
+    x = [1,2,3]
+    if cond:
+        x = [4,5,6]
+    if len(x): # this should be addressed
+        print(x)
+
+def g(cond:bool):
+    x = [1,2,3]
+    if cond:
+        x = [4,5,6] 
+    if len(x): # this should be addressed
+        print(x)
+    del x
+
+def h(cond:bool):
+    x = [1,2,3]
+    x = 123
+    if len(x): # ok
+        print(x)
+
+def outer():
+    x = [1,2,3]
+    def inner(x:int):
+        return x+1
+    if len(x): # [PLC1802]
+        print(x)
+
+def redefined():
+    x = 123
+    x = [1, 2, 3]
+    if len(x): # this should be addressed
+        print(x)
+
+global_seq = [1, 2, 3]
+
+def i():
+    global global_seq
+    if len(global_seq): # ok
+        print(global_seq)
+
+def j():
+    if False:
+        x = [1, 2, 3]
+    if len(x): # [PLC1802] should be fine
+        print(x)

crates/ruff_linter/src/checkers/ast/analyze/expression.rs

@@ -475,6 +475,9 @@ pub(crate) fn expression(expr: &Expr, checker: &mut Checker) {
             if checker.enabled(Rule::SuperWithoutBrackets) {
                 pylint::rules::super_without_brackets(checker, func);
             }
+            if checker.enabled(Rule::LenAsCondition) {
+                pylint::rules::len_as_condition(checker, call);
+            }
             if checker.enabled(Rule::BitCount) {
                 refurb::rules::bit_count(checker, call);
             }

crates/ruff_linter/src/codes.rs

@@ -191,6 +191,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pylint, "C0208") => (RuleGroup::Stable, rules::pylint::rules::IterationOverSet),
         (Pylint, "C0414") => (RuleGroup::Stable, rules::pylint::rules::UselessImportAlias),
         (Pylint, "C0415") => (RuleGroup::Preview, rules::pylint::rules::ImportOutsideTopLevel),
+        (Pylint, "C1802") => (RuleGroup::Preview, rules::pylint::rules::LenAsCondition),
         (Pylint, "C1901") => (RuleGroup::Preview, rules::pylint::rules::CompareToEmptyString),
         (Pylint, "C2401") => (RuleGroup::Stable, rules::pylint::rules::NonAsciiName),
         (Pylint, "C2403") => (RuleGroup::Stable, rules::pylint::rules::NonAsciiImportName),

crates/ruff_linter/src/rules/pylint/mod.rs

@@ -219,6 +219,7 @@ mod tests {
         Rule::BadStaticmethodArgument,
         Path::new("bad_staticmethod_argument.py")
     )]
+    #[test_case(Rule::LenAsCondition, Path::new("len_as_condition.py"))]
     fn rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!("{}_{}", rule_code.noqa_code(), path.to_string_lossy());
         let diagnostics = test_path(