robintw robintw - 2 years ago 371
Python Question

Apply function to each row of pandas dataframe to create two new columns

I have a pandas DataFrame,

containing multiple columns:

<class 'pandas.core.frame.DataFrame'>
DatetimeIndex: 53732 entries, 1993-01-07 12:23:58 to 2012-12-02 20:06:23
Data columns:
Date(dd-mm-yy)_Time(hh-mm-ss) 53732 non-null values
Julian_Day 53732 non-null values
AOT_1020 53716 non-null values
AOT_870 53732 non-null values
AOT_675 53188 non-null values
AOT_500 51687 non-null values
AOT_440 53727 non-null values
AOT_380 51864 non-null values
AOT_340 52852 non-null values
Water(cm) 51687 non-null values
%TripletVar_1020 53710 non-null values
%TripletVar_870 53726 non-null values
%TripletVar_675 53182 non-null values
%TripletVar_500 51683 non-null values
%TripletVar_440 53721 non-null values
%TripletVar_380 51860 non-null values
%TripletVar_340 52846 non-null values
440-870Angstrom 53732 non-null values
380-500Angstrom 52253 non-null values
440-675Angstrom 53732 non-null values
500-870Angstrom 53732 non-null values
340-440Angstrom 53277 non-null values
Last_Processing_Date(dd/mm/yyyy) 53732 non-null values
Solar_Zenith_Angle 53732 non-null values
dtypes: datetime64[ns](1), float64(22), object(1)

I want to create two new columns for this dataframe based on applying a function to each row of the dataframe. I don't want to have to call the function multiple times (eg. by doing two separate
calls) as it is rather computationally intensive. I have tried doing this in two ways, and neither of them work:


I have written a function which takes a
and returns a tuple of the values I want:

def calculate(s):
a = s['path'] + 2*s['row'] # Simple calc for example
b = s['path'] * 0.153
return (a, b)

Trying to apply this to the DataFrame gives an error:

st.apply(calculate, axis=1)
AssertionError Traceback (most recent call last)
<ipython-input-248-acb7a44054a7> in <module>()
----> 1 st.apply(calculate, axis=1)

C:\Python27\lib\site-packages\pandas\core\frame.pyc in apply(self, func, axis, broadcast, raw, args, **kwds)
4191 return self._apply_raw(f, axis)
4192 else:
-> 4193 return self._apply_standard(f, axis)
4194 else:
4195 return self._apply_broadcast(f, axis)

C:\Python27\lib\site-packages\pandas\core\frame.pyc in _apply_standard(self, func, axis, ignore_failures)
4274 index = None
-> 4276 result = self._constructor(data=results, index=index)
4277 result.rename(columns=dict(zip(range(len(res_index)), res_index)),
4278 inplace=True)

C:\Python27\lib\site-packages\pandas\core\frame.pyc in __init__(self, data, index, columns, dtype, copy)
390 mgr = self._init_mgr(data, index, columns, dtype=dtype, copy=copy)
391 elif isinstance(data, dict):
--> 392 mgr = self._init_dict(data, index, columns, dtype=dtype)
393 elif isinstance(data, ma.MaskedArray):
394 mask = ma.getmaskarray(data)

C:\Python27\lib\site-packages\pandas\core\frame.pyc in _init_dict(self, data, index, columns, dtype)
522 return _arrays_to_mgr(arrays, data_names, index, columns,
--> 523 dtype=dtype)
525 def _init_ndarray(self, values, index, columns, dtype=None,

C:\Python27\lib\site-packages\pandas\core\frame.pyc in _arrays_to_mgr(arrays, arr_names, index, columns, dtype)
5412 # consolidate for now
-> 5413 mgr = BlockManager(blocks, axes)
5414 return mgr.consolidate()

C:\Python27\lib\site-packages\pandas\core\internals.pyc in __init__(self, blocks, axes, do_integrity_check)
803 if do_integrity_check:
--> 804 self._verify_integrity()
806 self._consolidate_check()

C:\Python27\lib\site-packages\pandas\core\internals.pyc in _verify_integrity(self)
892 "items")
893 if block.values.shape[1:] != mgr_shape[1:]:
--> 894 raise AssertionError('Block shape incompatible with manager')
895 tot_items = sum(len(x.items) for x in self.blocks)
896 if len(self.items) != tot_items:

AssertionError: Block shape incompatible with manager

I was then going to assign the values returned from
to two new columns using the method shown in this question. However, I can't even get to this point! This all works fine if I just return one value.

Using a loop:

I first created two new columns of the dataframe and set them to

st['a'] = None
st['b'] = None

Then looped over all of the indices and tried to modify these
values that I'd got in there, but the modifications I did didn't seem to work. That is, no error was generated, but the DataFrame didn't seem to be modified.

for i in st.index:
# do calc here
st.ix[i]['a'] = a
st.ix[i]['b'] = b

I thought that both of these methods would work, but neither of them did. So, what am I doing wrong here? And what is the best, most 'pythonic' and 'pandaonic' way to do this?

Answer Source

To make the first approach work, try returning a Series instead of a tuple (apply is throwing an exception because it doesn't know how to glue the rows back together as the number of columns doesn't match the original frame).

def calculate(s):
    a = s['path'] + 2*s['row'] # Simple calc for example
    b = s['path'] * 0.153
    return pd.Series(dict(col1=a, col2=b))

The second approach should work if you replace:

st.ix[i]['a'] = a


st.ix[i, 'a'] = a
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