Python pytest: local variable 'length' referenced before assignment

Question

I am not sure what is wrong in the following code snippet.

I have the following two versions of a function.

Version 1

def _check_array_lengths(self, data):
    for i, values in data.items():
        if i == 0:
            length = len(values)
        if length != len(values):
            raise ValueError('All values must be the same length')

When I run test, the above function fails with a msg
"ERROR tests/test_dataframe.py – UnboundLocalError: local variable ‘length’ referenced before assignment"

Version 2

def _check_array_lengths(self, data):
    for i, values in enumerate(data.values()):
        if i == 0:
            length = len(values)
        if length != len(values):
            raise ValueError('All values must be the same length')

The test for this function works fine and I wonder why I don’t see the same error msg(mentioned above) here. How that "enumerate" is causing this change in behavior!

May be something really silly but I couldn’t figure it out yet.

Here is my test function

def test_array_length(self):
    with pytest.raises(ValueError):
        pdc.DataFrame({'a': np.array([1, 2]), 
                       'b': np.array([1])})

can you please help ?

Asked By: user19

||

Source

Answer 1

In the second version, the first value of i is guaranteed to be 0. So the condition if i == 0: will be true, and length will be set. Then the comparison if length != len(values): will be able to use the length variable.

In the first version is i iterates over the dataframe indexes, not a numeric sequence. The values of i will be 'a' and 'b'. The if i == 0: condition will never be true, so you never set length, and get an error when you try to compare it.

Answered By: Barmar

Python pytest: local variable 'length' referenced before assignment

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