pandas - When to apply(pd.to_numeric) and when to astype(np.float64) in python?

Question

I have a pandas DataFrame object named xiv which has a column of int64 Volume measurements.

In[]: xiv['Volume'].head(5)
Out[]: 

0    252000
1    484000
2     62000
3    168000
4    232000
Name: Volume, dtype: int64

I have read other posts (like this and this) that suggest the following solutions. But when I use either approach, it doesn't appear to change the dtype of the underlying data:

In[]: xiv['Volume'] = pd.to_numeric(xiv['Volume'])

In[]: xiv['Volume'].dtypes
Out[]: 
dtype('int64')

Or...

In[]: xiv['Volume'] = pd.to_numeric(xiv['Volume'])
Out[]: ###omitted for brevity###

In[]: xiv['Volume'].dtypes
Out[]: 
dtype('int64')

In[]: xiv['Volume'] = xiv['Volume'].apply(pd.to_numeric)

In[]: xiv['Volume'].dtypes
Out[]: 
dtype('int64')

I've also tried making a separate pandas Series and using the methods listed above on that Series and reassigning to the x['Volume'] obect, which is a pandas.core.series.Series object.

I have, however, found a solution to this problem using the numpy package's float64 type - this works but I don't know why it's different.

In[]: xiv['Volume'] = xiv['Volume'].astype(np.float64)

In[]: xiv['Volume'].dtypes
Out[]: 
dtype('float64') 

Can someone explain how to accomplish with the pandas library what the numpy library seems to do easily with its float64 class; that is, convert the column in the xiv DataFrame to a float64 in place.

Answer 1

If you already have numeric dtypes (int8|16|32|64,float64,boolean) you can convert it to another "numeric" dtype using Pandas .astype() method.

Demo:

In [90]: df = pd.DataFrame(np.random.randint(10**5,10**7,(5,3)),columns=list('abc'), dtype=np.int64)

In [91]: df
Out[91]:
         a        b        c
0  9059440  9590567  2076918
1  5861102  4566089  1947323
2  6636568   162770  2487991
3  6794572  5236903  5628779
4   470121  4044395  4546794

In [92]: df.dtypes
Out[92]:
a    int64
b    int64
c    int64
dtype: object

In [93]: df['a'] = df['a'].astype(float)

In [94]: df.dtypes
Out[94]:
a    float64
b      int64
c      int64
dtype: object

It won't work for object (string) dtypes, that can't be converted to numbers:

In [95]: df.loc[1, 'b'] = 'XXXXXX'

In [96]: df
Out[96]:
           a        b        c
0  9059440.0  9590567  2076918
1  5861102.0   XXXXXX  1947323
2  6636568.0   162770  2487991
3  6794572.0  5236903  5628779
4   470121.0  4044395  4546794

In [97]: df.dtypes
Out[97]:
a    float64
b     object
c      int64
dtype: object

In [98]: df['b'].astype(float)
...
skipped
...
ValueError: could not convert string to float: 'XXXXXX'

So here we want to use pd.to_numeric() method:

In [99]: df['b'] = pd.to_numeric(df['b'], errors='coerce')

In [100]: df
Out[100]:
           a          b        c
0  9059440.0  9590567.0  2076918
1  5861102.0        NaN  1947323
2  6636568.0   162770.0  2487991
3  6794572.0  5236903.0  5628779
4   470121.0  4044395.0  4546794

In [101]: df.dtypes
Out[101]:
a    float64
b    float64
c      int64
dtype: object