False positive with E1136 unsubscriptable-object for use in `or` after `is None` guard

[sam-s]

Steps to reproduce

1. Create file z.py

a = None
while True:
    if a is None or a["1"] == 0:
        a = {"1": 1}
    else:
        break
print("Done")

2. run pylint z.py

Current behavior

E:  3,20: Value 'a' is unsubscriptable (unsubscriptable-object)

Expected behavior

no errors, no warnings

pylint --version output

pylint 1.7.1, 
astroid 1.5.2
Python 2.7.13 (default, Dec 18 2016, 07:03:39) 
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.42.1)]

[erjiang]

This has hit us too. We are using a variable that is initialized to None, but then gets assigned to dict in a loop.

I think something like this will trigger it too:

points = [{"lat": 39, "lon": -92}, {"lat": 39.1, "lon": -92.1}]

furthest_west = None
for p in points:
    if furthest_west is None:
        furthest_west = p
    else:
        if p['lon'] < furthest_west['lon']:
            furthest_west = p

not sure how you would solve this, seems like the inferencer would have to go into the if/else clauses to see what might get assigned to the variable to figure out that it's Union(None, dict).

[hughes]

Even checking isinstance(thing, list) immediately prior to using thing will trigger this.

[PCManticore]

Yes @hughes that's a problem that's still crippling pylint even today (but hopefully with the new number of committers we have, we might tackle it at some point). I'm referring here to the fact that pylint does not quite grasp control flow, that is, if you use isinstance, pylint will happily infer all potential values that a variable could have, disregarding the branch hints.

[ch3ck]

I had this problem today opening a config file.

[kelvinau]

+1

[sdmunozsierra]

I have this false positive too. I reference a variable instanced as none in the main object class, and then change it in function from an inherited class.
Code example below; incomplete for brevity.

class Response():
    def __init__(self):
        self.raw_response = None

    def filter_raw_response(self):
        if not self.raw_response:
            return None
        num_datapoints = len(self.raw_response['Datapoints'])   # Error shows here
        if num_datapoints == 0:
            return None

class SpecificResponse(Response):

    def craft_response(self)
        # Variable instanced here:
        self.raw_response = self.client.get_metric_statistics()

[jiarongqiu]

+1

I got this too when I tried to iterate over candidates, a matrix like variable. It reports item as NoneType and raise E1136 error. But, actually, it is a list.

candidates = [item for item in val_dataset if item[-1] == i]
candidates = random.sample(candidates,100)
for item in candidates:
    img_path = os.path.join(data_dir,item[0])
    out_path = os.path.join(output_folder,str(i))
    os.system('cp %s %s'%(img_path,out_path))

[lucmos]

False positive even if the custom class implements the __getitem__ method.

[melyux]

Will this ever be fixed then?

[PCManticore]

@melikyuksel Unless someone proposes a patch, it's unlikely.