Why prompt me “name 'pd' is not defined”
### Question about pandas

I have customized a Python interpreter, on which I try to run pandas's apply method, but when executing this method, there will be an exception. I don't know why, who knows why

```python

[1] import pandas as pd
[2] titanic = pd.read_csv('../data/titanic.csv')
[3] def not_null_count(columns):
             return len(columns[pd.isnull(columns)])
[4] res = titanic.apply(not_null_count)
[5]print(res)

run_code:
comp_code = compile(code, '<zmq-kernel>', 'single')
            if comp_code:
                exec(comp_code, self.user_global_ns, self.user_ns)

```
File "<zmq-kernel>", line 1, in <module>
File "/usr/local/anaconda3/lib/python3.8/site-packages/pandas/core/frame.py", line 6878, in apply return op.get_result()
File "/usr/local/anaconda3/lib/python3.8/site-packages/pandas/core/apply.py", line 186, in get_result return self.apply_standard()
File "/usr/local/anaconda3/lib/python3.8/site-packages/pandas/core/apply.py", line 295, in apply_standard result = libreduction.compute_reduction(
File "pandas/_libs/reduction.pyx", line 620, in pandas._libs.reduction.compute_reduction
File "pandas/_libs/reduction.pyx", line 128, in pandas._libs.reduction.Reducer.get_result
File "<zmq-kernel>", line 2, in not_null_count
NameError: name 'pd' is not defined