Exception while comparing empty strings

[akoshulian-exos]

Minimal, reproducible example:

def test_frame(dataframe_regression):
    df = pd.DataFrame.from_records([
        {
            'a': 'a',
            'b': 'b'
        },
        {
            'a': 'a1',
            'b': ''
        },
    ])

    dataframe_regression.check(df)

This test results in rising the following exception:
{TypeError}unsupported operand type(s) for -: 'str' and 'str'

The issue arises because the empty string is converted to nan while reading a csv file containing the dataframe.

Traceback (most recent call last):
  File ".../venv/lib/python3.7/site-packages/IPython/core/interactiveshell.py", line 3437, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)
  File "<ipython-input-1-b86ebfe209ab>", line 1, in <module>
    dataframe_regression.check(df)
  File ".../venv/lib/python3.7/site-packages/pytest_regressions/dataframe_regression.py", line 260, in check
    force_regen=self._force_regen,
  File ".../venv/lib/python3.7/site-packages/pytest_regressions/common.py", line 153, in perform_regression_check
    check_fn(obtained_filename, filename)
  File ".../venv/lib/python3.7/site-packages/pytest_regressions/dataframe_regression.py", line 143, in _check_fn
    diffs = np.abs(obtained_column - expected_column)[diff_ids]
  File ".../venv/lib/python3.7/site-packages/pandas/core/ops/common.py", line 65, in new_method
    return method(self, other)
  File ".../venv/lib/python3.7/site-packages/pandas/core/arraylike.py", line 97, in __sub__
    return self._arith_method(other, operator.sub)
  File ".../venv/lib/python3.7/site-packages/pandas/core/series.py", line 4998, in _arith_method
    result = ops.arithmetic_op(lvalues, rvalues, op)
  File ".../venv/lib/python3.7/site-packages/pandas/core/ops/array_ops.py", line 189, in arithmetic_op
    res_values = _na_arithmetic_op(lvalues, rvalues, op)
  File ".../venv/lib/python3.7/site-packages/pandas/core/ops/array_ops.py", line 149, in _na_arithmetic_op
    result = _masked_arith_op(left, right, op)
  File ".../venv/lib/python3.7/site-packages/pandas/core/ops/array_ops.py", line 91, in _masked_arith_op
    result[mask] = op(xrav[mask], yrav[mask])
TypeError: unsupported operand type(s) for -: 'str' and 'str'

[nicoddemus]

Thanks @akoshulian-exos for the report!