Why would you get NoneType error in python even when explicitly checking if None?

Question

I was recently trying to read in a csv, of which one of the columns is serialized as list strings. Using ast.literal_eval usually converts the string to an actual list. In this case, even though we check whether a is a list or not, we get back an error saying None type is non subscriptable.

Code:

def generate(name):
    df = pd.read_csv(name)
    df['column'] = df['column'].apply(lambda x: literal_eval((x))) # all x should become list
    for a in df['column']:
        if a and isinstance(a, list):
            print(a)[0]
        else:
            print('jey')

Error message (the first list is printed)

[0.0, 0.0, 0.014, 0.0]
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-34-d84f295beae6> in <module>
----> 1 generate('parameter_vv/perimeter_log.csv')

<ipython-input-33-e9ee10a439d4> in generate(name)
      4     for a in df['column']:
      5         if a and isinstance(a, list):
----> 6             print(a)[0]
      7         else:
      8             print('jey')

TypeError: 'NoneType' object is not subscriptable

print returns None. I think you wanted print(a[0])

hurlenko
– hurlenko

2020-01-30 16:19:20 +00:00
Commented Jan 30, 2020 at 16:19 — hurlenko
– hurlenko, Commented Jan 30, 2020 at 16:19

neverwalkaloner · Accepted Answer · 2020-01-30 16:19:34Z

1

print(a) return None and you see this error when try to do print(a)[0].

I suppose you want to do this instead:

print(a[0])

answered Jan 30, 2020 at 16:19

neverwalkaloner

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Comments

Valentin M. · Accepted Answer · 2020-01-30 16:20:55Z

1

You have to use print(a[0]) instead of print(a)[0].

print() returns None, so print(a)[0] try to get element 0 from None, this is the error you get.

answered Jan 30, 2020 at 16:20

Valentin M.

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