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I am stuck with my pandas script.

Actually , i am working with two csv file(one input and the other output file). i want to copy all the rows of two column and want to make calculation and then copy it to another dataframe (output file).

The columns are as follows :

'lat', 'long','PHCount', 'latOffset_1', 'longOffset_1','PH_Lat_1', 'PH_Long_1', 'latOffset_2', 'longOffset_2', 'PH_Lat_2', 'PH_Long_2', 'latOffset_3', 'longOffset_3','PH_Lat_3', 'PH_Long_3',  'latOffset_4', 'longOffset_4','PH_Lat_4', 'PH_Long_4'.

i want to take the column 'lat' and 'latOffset_1' , do some calculation and put it in another new column('PH_Lat_1') which i have already created.

My function is :

def calculate_latoffset(latoffset):  #Calculating Lat offset.
    a=(df2['lat']-(2*latoffset))
    return a

The main code :

for i in range(1,5):
        print(i)
        a='PH_lat_%d' % i 
        print (a)
        b='latOffset_%d' % i
        print (b)
        df2.a = df2.apply(lambda x: calculate_latoffset(x[b]), axis=1)

Since the column name just differ by (1,2,3,4). so i want to call the function calculate_latoffset and calculate the all the rows of all the columns(PH_Lat_1, PH_Lat_2, PH_Lat_3,PH_Lat_4) in one go.

When using the above code i am getting this error :

basic_conversion.py:46: UserWarning: Pandas doesn't allow columns to be created via a new attribute name - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access
  df2.a = df2.apply(lambda x: calculate_latoffset(x[b]), axis=1)

is it possible ? Please kindly help

YaOzI
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Bushra Ghazal
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5 Answers5

53

Simply use df2['a'] instead of df2.a

AaronDT
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11

This is a Warning not an Error, so your code could still run through, but probably not following your intention.

  1. Short answer: To create a new column for DataFrame, never use attribute access, the correct way is to use either [] or .loc indexing:

    >>> df
   a  b
0  7  6
1  5  8
>>> df['c'] = df.a + df.b 
>>> # OR
>>> df.loc[:, 'c'] = df.a + df.b
>>> df # c is an new added column
   a  b   c
0  7  6  13
1  5  8  13

More explaination, Seires and DataFrame are core classes and data structures in pandas, and of course they are Python classes too, so there are some minor distinction when involving attribute access between pandas DataFrame and normal Python objects. But it's well documented and can be easily understood. Just few points to note:

  1. In Python, users may dynamically add data attributes of their own to an instance object using attribute access.

    >>> class Dog(object):
...     pass
>>> dog = Dog()
>>> vars(dog)
{}
>>> superdog = Dog()
>>> vars(superdog)
{}
>>> dog.legs = 'I can run.'
>>> superdog.wings = 'I can fly.'
>>> vars(dog)
{'legs': 'I can run.'}
>>> vars(superdog)
{'wings': 'I can fly.'}

  2. In pandas, index and column are closely related to the data structure, you may access an index on a Series, column on a DataFrame as an attribute.

    >>> import pandas as pd
>>> import numpy as np
>>> data = np.random.randint(low=0, high=10, size=(2,2))
>>> df = pd.DataFrame(data, columns=['a', 'b'])
>>> df
   a  b
0  7  6
1  5  8
>>> vars(df)
{'_is_copy': None, 
 '_data': BlockManager
    Items: Index(['a', 'b'], dtype='object')
    Axis 1: RangeIndex(start=0, stop=2, step=1)
    IntBlock: slice(0, 2, 1), 2 x 2, dtype: int64,
 '_item_cache': {}}

  3. But, pandas attribute access is mainly a convinience for reading from and modifying an existing element of a Series or column of a DataFrame.

    >>> df.a
0    7
1    5
Name: a, dtype: int64
>>> df.b = [1, 1]
>>> df
   a  b
0  7  1
1  5  1

  4. And, the convinience is a tradeoff for full functionality. E.g. you can create a DataFrame object with column names ['space bar', '1', 'loc', 'min', 'index'], but you can't access them as an attribute, because they are either not a valid Python identifier 1, space bar or conflicts with an existing method name.

    >>> data = np.random.randint(0, 10, size=(2, 5))
>>> df_special_col_names = pd.DataFrame(data, columns=['space bar', '1', 'loc', 'min', 'index'])
>>> df_special_col_names
   space bar  1  loc  min  index
0          4  4    4    8      9
1          3  0    1    2      3

  5. In these cases, the .loc, .iloc and [] indexing is the defined way to fullly access/operate index and columns of Series and DataFrame objects.

    >>> df_special_col_names['space bar']
0    4
1    3
Name: space bar, dtype: int64
>>> df_special_col_names.loc[:, 'min']
0    8
1    2
Name: min, dtype: int64
>>> df_special_col_names.iloc[:, 1]
0    4
1    0
Name: 1, dtype: int64

  6. As to the topic, to create a new column for DataFrame, as you can see, df.c = df.a + df.b just created an new attribute along side to the core data structure, so starting from version 0.21.0 and later, this behavior will raise a UserWarning (silent no more).

    >>> df
   a  b
0  7  1
1  5  1
>>> df.c = df.a + df.b
__main__:1: UserWarning: Pandas doesn't allow columns to be created via a new attribute name - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access
>>> df['d'] = df.a + df.b
>>> df
   a  b  d
0  7  1  8
1  5  1  6
>>> df.c
0    8
1    6
dtype: int64
>>> vars(df)
{'_is_copy': None, 
 '_data': 
    BlockManager
    Items: Index(['a', 'b', 'd'], dtype='object')
    Axis 1: RangeIndex(start=0, stop=2, step=1)
    IntBlock: slice(0, 2, 1), 2 x 2, dtype: int64
    IntBlock: slice(2, 3, 1), 1 x 2, dtype: int64, 
 '_item_cache': {},
 'c': 0    8
      1    6
      dtype: int64}

  7. Finally, back to the Short answer.

YaOzI
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4

The solution I can think of is to use .loc to get the column. You can try df.loc[:,a] instead of df.a. Pandas dataframe columns cannot be created using the dot method to avoid potential conflicts with the dataframe attributes. Hope this helps

Ying Cai
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1

although all other answers are likely a much better solution, i figured that it does no harm to just ignore it move on.

import warnings
warnings.filterwarnings("ignore","Pandas doesn't allow columns to be created via a new attribute name - see https://pandas.pydata.org/pandas-docs/stable/indexing.html#attribute-access", UserWarning)

using the code above, the script will just disregard the warning and move on.

Alice Carvalho
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0

In df2.apply(lambda x: calculate_latoffset(x[b]), axis=1) you are creating a 5 column dataframe and you were trying to assign the value to a single field. Do df2[a] = calculate_latoffset(df2[b]) instead should deliver the desired output.

bowei
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