如何使用python（pandas）更新csv文件中所有行的最后一列值

Question

I am trying to update last column value for all the rows in the csv file using Pandas. but while updating the value, other column value are dropping.

file = r'Test.csv'
# Read the file
df = pd.read_csv(file, error_bad_lines=False)
# df.at[3, "ingestion"] = '20'
df.set_value(1, "ingestion", '30')
df.to_csv("Test.csv", index=False, sep='|')

Answer 1

Use DataFrame.iloc with -1 for select last column and : for select all rows:

df = pd.DataFrame({'A':list('abcdef'),
                   'B':[4,5,4,5,5,4],
                   'C':[7,8,9,4,2,3],
                   'D':[1,3,5,7,1,0],
                   'E':[5,3,6,9,2,4],
                   'F':list('aaabbb')})

print (df)
   A  B  C  D  E  F
0  a  4  7  1  5  a
1  b  5  8  3  3  a
2  c  4  9  5  6  a
3  d  5  4  7  9  b
4  e  5  2  1  2  b
5  f  4  3  0  4  b

df.iloc[:, -1] = '20'
print (df)
   A  B  C  D  E   F
0  a  4  7  1  5  20
1  b  5  8  3  3  20
2  c  4  9  5  6  20
3  d  5  4  7  9  20
4  e  5  2  1  2  20
5  f  4  3  0  4  20

EDIT:

For update all rows by last edit value swap -1 with : and get last column value by DataFrame.iat :

df.iloc[-1, :] = df.iat[-1, -1]
print (df)
   A  B  C  D  E  F
0  a  4  7  1  5  a
1  b  5  8  3  3  a
2  c  4  9  5  6  a
3  d  5  4  7  9  b
4  e  5  2  1  2  b
5  b  b  b  b  b  b

Answer 2

pd.DataFrame.set_value is not appropriate for setting all the values in a column. As per the docs, it is used to setting a scalar at a specific row and column label combination.

Moreover, since v0.21, it has been deprecated in favour of .at / .iat accessors.

Instead, you can set the value directly by extracting the final column label, assuming you have no duplicate column names:

df[df.columns[-1]] = '20'

Or, more directly, you can use the iloc accessor:

df.iloc[:, -1] = '20'