我正在编写一个简单的代码，但是出现此错误，请帮助解决此问题。
import math
import pandas as pd
import matplotlib.pyplot as plt
import matplotlib
from sklearn import preprocessing
df= pd.read_html('https://github.com/authman/DAT210x/blob/master/Module4/Datasets/kidney_disease.csv')[0]
# drop id column
df.drop('id', axis = 1, inplace = True)
#Get rid of any row that has at least 1 NANs in it ## Drop the rows where any of the elements is nan
df.dropna(axis = 0, how = 'any', inplace = True)
# Use an indexer to select only the following columns:
# ['bgr','wc','rc']
df = df[['bgr', 'wc', 'rc']]
df.dtypes
bgr object
wc object
rc object
dtype: object
df.head(5)
bgr wc rc
0 notpresent 32 6700
1 notpresent 29 12100
2 notpresent 32 4500
3 present 16 11000
4 notpresent 24 9200
# Now they are all floats
df.bgr = pd.to_numeric(df.bgr, downcast='float')
df.wc = pd.to_numeric(df.wc, downcast='float')
df.rc = pd.to_numeric(df.rc, downcast='float')
---------------------------------------------------------------------------
ValueError Traceback (most recent call last)
pandas\src\inference.pyx in pandas.lib.maybe_convert_numeric (pandas\lib.c:55708)()
ValueError: Unable to parse string "notpresent"
During handling of the above exception, another exception occurred:
ValueError Traceback (most recent call last)
<ipython-input-246-24d33f210233> in <module>()
----> 1 df.bgr = pd.to_numeric(df.bgr, downcast='float')
2 df.wc = pd.to_numeric(df.wc, downcast='float')
3 df.rc = pd.to_numeric(df.rc, downcast='float')
C:\Users\hp\Anaconda3\lib\site-packages\pandas\tools\util.py in to_numeric(arg, errors, downcast)
193 coerce_numeric = False if errors in ('ignore', 'raise') else True
194 values = lib.maybe_convert_numeric(values, set(),
--> 195 coerce_numeric=coerce_numeric)
196
197 except Exception:
pandas\src\inference.pyx in pandas.lib.maybe_convert_numeric (pandas\lib.c:56097)()
ValueError: Unable to parse string "notpresent" at position 0