Why some commands cause an error relative to the preceding command?

Question

This is a pandas question.

Try to copy this in Jupyter Notebook:

In [1]: df = pd.DataFrame([[1, 2], [4, 5], [7, 8]],
        index=['cobra', 'viper', 'sidewinder'],
        columns=['max_speed', 'shield'])

        df

In [2]: df.pop('shield') # Return as series.

In [3]: pd.DataFrame(df.pop('shield')) # Return as DataFrame.

Then inverse it to the sequence of

In[1]
In[3]
In[2]

Why the 3rd Out[-] always cause an error?

I oftentimes encounter this kinds of error. Is this a cache issue? Redundancy? What is the reason why such error occurs?

Answer 1

I think your code generate expected error:

df = pd.DataFrame([[1, 2], [4, 5], [7, 8]],
        index=['cobra', 'viper', 'sidewinder'],
        columns=['max_speed', 'shield'])

Here DataFrame.pop take column shield from original Dataframe, create Series and drop from original:

a = df.pop('shield') # Return as series.
print (a)
cobra         2
viper         5
sidewinder    8
Name: shield, dtype: int64

So no column in df after pop:

print (df)
            max_speed
cobra               1
viper               4
sidewinder          7

So failed get column shield, because not exist in df:

b = pd.DataFrame(df.pop('shield')) # Return as DataFrame.
print (b)
KeyError: 'shield'