[英]Python: use a function in pandas lambda expression
I have the following code, trying to find the hour of the 'Dates' column in a data frame: 我有以下代码,试图找到数据框中“日期”列的小时:
print(df['Dates'].head(3))
df['hour'] = df.apply(lambda x: find_hour(x['Dates']), axis=1)
def find_hour(self, input):
return input[11:13].astype(float)
where the print(df['Dates'].head(3))
looks like: print(df['Dates'].head(3))
如下所示:
0 2015-05-13 23:53:00
1 2015-05-13 23:53:00
2 2015-05-13 23:33:00
However, I got the following error: 但是,我收到以下错误:
df['hour'] = df.apply(lambda x: find_hour(x['Dates']), axis=1)
NameError: ("global name 'find_hour' is not defined", u'occurred at index 0')
Does anyone know what I missed? 有谁知道我错过了什么? Thanks!
谢谢!
Note that if I put the function directly in the lambda line like below, everything works fine: 请注意,如果我将函数直接放在lambda行中,如下所示,一切正常:
df['hour'] = df.apply(lambda x: x['Dates'][11:13], axis=1).astype(float)
You are trying to use find_hour
before it has yet been defined. 您在尝试使用
find_hour
之前尝试使用它。 You just need to switch things around: 你只需要切换一下:
def find_hour(self, input):
return input[11:13].astype(float)
print(df['Dates'].head(3))
df['hour'] = df.apply(lambda x: find_hour(x['Dates']), axis=1)
Edit : Padraic has pointed out a very important point: find_hour()
is defined as taking two arguments, self
and input
, but you are giving it only one. 编辑 :Padraic指出了一个非常重要的观点:
find_hour()
被定义为接受两个参数, self
和input
,但是你只给它一个。 You should define find_hour()
as def find_hour(input):
except that defining the argument as input
shadows the built-in function. 您应该将
find_hour()
定义为def find_hour(input):
除了将参数定义为input
影响内置函数。 You might consider renaming it to something a little more descriptive. 您可以考虑将其重命名为更具描述性的内容。
what is wrong with old good .dt.hour
? 旧的好
.dt.hour
什么问题?
In [202]: df
Out[202]:
Date
0 2015-05-13 23:53:00
1 2015-05-13 23:53:00
2 2015-05-13 23:33:00
In [217]: df['hour'] = df.Date.dt.hour
In [218]: df
Out[218]:
Date hour
0 2015-05-13 23:53:00 23
1 2015-05-13 23:53:00 23
2 2015-05-13 23:33:00 23
and if your Date
column is of string type you may want to convert it to datetime first : 如果你的
Date
列是字符串类型的,你可能需要先将其转换成datetime:
df.Date = pd.to_datetime(df.Date)
or just: 要不就:
df['hour'] = int(df.Date.str[11:13])
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