python - 更新表中数据的正确方法？

Question

我需要从现有数据中在 pandas 数据框中添加三列。

df
>>
    n    a     b
0   3  1.2   1.4
1   2  2.8   3.8
2   3  2.3   2.0
3   3  1.7   5.7
4   2  6.9   4.9
5   1  3.9  19.0
6   9  2.3   8.3
7   5  8.5   3.1
8  18  6.7   7.0
9  10  5.6   6.4

我做了以下事情

import pandas
import numpy
def add_tests(add_df):
    new_tests = """
        (a+b)/n
        (a*b)/n
        ((a+b)/n)**-1
    """.split()
    rows = add_df.shape[0]
    cols = len(new_tests)
    U = pandas.DataFrame(numpy.empty([rows, cols]), columns=new_tests)
    add_df = pandas.concat([df, U], axis=1)
    for i, row in add_df.iterrows():
        # 1) good calculation:
        add_df['(a+b)/n'].loc[i] = (add_df['a'].loc[i] + add_df['b'].loc[i])/ add_df['n'].loc[i]
        # 2) good calculation (Both ways):
        add_df['(a*b)/n'].loc[i] = (row['a'] * row['b'])/ row['n']
        # 3) bad calculation
        add_df['((a+b)/n)**-1'].loc[i] = row['(a+b)/n'] ** -1
        pass
    return add_df

我收到下一条警告消息:

df = add_tests(df)
df
>>

C:...\pandas\core\indexing.py:141: SettingWithCopyWarning: A value is trying to be set on a copy of a slice from a DataFrame

    n    a     b    (a+b)/n    (a*b)/n  ((a+b)/n)**-1
0   3  1.2   1.4   0.866667   0.560000       0.833333
1   2  2.8   3.8   3.300000   5.320000       0.588235
2   3  2.3   2.0   1.433333   1.533333       0.434783
3   3  1.7   5.7   2.466667   3.230000       0.178571
4   2  6.9   4.9   5.900000  16.905000       0.500000
5   1  3.9  19.0  22.900000  74.100000       0.052632
6   9  2.3   8.3   1.177778   2.121111       0.142857
7   5  8.5   3.1   2.320000   5.270000       0.263158
8  18  6.7   7.0   0.761111   2.605556       0.111111
9  10  5.6   6.4   1.200000   3.584000       0.666667

显然第3步无法正常工作......如何正确地做到这一点？

Answer 1

eval的乐趣

• 使用公式定义临时列名称的元组
• 创建一个 \n 分隔的公式字符串以传递给 eval
• 使用字典将公式做成列名

<小时/>

ftups = [('aa', '(a+b)/n'), ('bb', '(a*b)/n'), ('cc', '((a+b)/n)**-1')]
forms = '\n'.join([' = '.join(tup) for tup in ftups])
fdict = dict(ftups)

df.eval(forms, inplace=False).rename(columns=fdict)

    n    a     b    (a+b)/n    (a*b)/n  ((a+b)/n)**-1
0   3  1.2   1.4   0.866667   0.560000       1.153846
1   2  2.8   3.8   3.300000   5.320000       0.303030
2   3  2.3   2.0   1.433333   1.533333       0.697674
3   3  1.7   5.7   2.466667   3.230000       0.405405
4   2  6.9   4.9   5.900000  16.905000       0.169492
5   1  3.9  19.0  22.900000  74.100000       0.043668
6   9  2.3   8.3   1.177778   2.121111       0.849057
7   5  8.5   3.1   2.320000   5.270000       0.431034
8  18  6.7   7.0   0.761111   2.605556       1.313869
9  10  5.6   6.4   1.200000   3.584000       0.833333