Pandas 从日期类型列中获取星期几
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Pandas Get Day of Week from date type column
提问by kdragger
I'm using Python 3.6 and Pandas 0.20.3.
我使用的是 Python 3.6 和 Pandas 0.20.3。
I have a column that I've converted to date type from datetime. All I need is the date. I have it as a derived column for ease of use. But I'm looking to do some further operations via a day of the week calculation. I can get the day of week from a datetime type but not from the date. It seems to me that this should be possible but I've tried multiple variations and not found success.
我有一个从日期时间转换为日期类型的列。我只需要日期。为了便于使用，我将其作为派生列。但我希望通过一周中的一天计算做一些进一步的操作。我可以从日期时间类型中获取星期几，但不能从日期中获取。在我看来，这应该是可能的，但我尝试了多种变体，但都没有成功。
Here is an example:
下面是一个例子：
import numpy as np
import pandas as pd
df = pd.DataFrame({'date':['2017-5-16','2017-5-17']})
df['trade_date']=pd.to_datetime(df['date'])
I can get the day of the week from the datetime column 'trade_date'.
我可以从日期时间列“trade_date”中获取星期几。
df['dow']=df['trade_date'].dt.dayofweek
df
date trade_date dow
0 2017-5-16 2017-05-16 1
1 2017-5-17 2017-05-17 2
But if I have a date, rather than a datetime, no dice: For instance:
但是如果我有一个日期，而不是一个日期时间，没有骰子：例如：
df['trade_date_2']=pd.to_datetime(df['date']).dt.date
And then:
进而：
df['dow_2']=df['trade_date_2'].dt.dayofweek
I get (at the end):
我得到（最后）：
AttributeError: Can only use .dt accessor with datetimelike values
I've tried various combinations of dayofweek(), weekday, weekday() which, I realize, highlight my ignorance of exactly how Pandas works. So ... any suggestions besides adding another column which is the datetime version of column trade_date? I'll also welcome explanations of why this is not working.
我尝试了 dayofweek()、weekday、weekday() 的各种组合，我意识到，这凸显了我对 Pandas 工作原理的无知。所以......除了添加另一列是列trade_date的日期时间版本之外还有什么建议吗？我也欢迎解释为什么这不起作用。
回答by jezrael
There is problem it is difference between pandas datetime(timestamps) where are implemented .dtmethods and python datewhere not.
问题在于pandas datetime（时间戳）在何处实现.dt方法和python date在何处不实现方法之间存在差异。
#return python date
df['trade_date_2']= pd.to_datetime(df['date']).dt.date
print (df['trade_date_2'].apply(type))
0 <class 'datetime.date'>
1 <class 'datetime.date'>
Name: trade_date_2, dtype: object
#cannot work with python date
df['dow_2']=df['trade_date_2'].dt.dayofweek
Need convert to pandas datetime:
需要转换为pandas datetime：
df['dow_2']= pd.to_datetime(df['trade_date_2']).dt.dayofweek
print (df)
date trade_date_2 dow_2
0 2017-5-16 2017-05-16 1
1 2017-5-17 2017-05-17 2
So the best is use:
所以最好是使用：
df['date'] = pd.to_datetime(df['date'])
print (df['date'].apply(type))
0 <class 'pandas._libs.tslib.Timestamp'>
1 <class 'pandas._libs.tslib.Timestamp'>
Name: date, dtype: object
df['trade_date_2']= df['date'].dt.date
df['dow_2']=df['date'].dt.dayofweek
print (df)
date trade_date_2 dow_2
0 2017-05-16 2017-05-16 1
1 2017-05-17 2017-05-17 2
EDIT:
编辑：
Thank you Bharath shettyfor solution working with python date- failed with NaT:
感谢Bharath shetty提供的解决方案python date- 失败NaT：
df = pd.DataFrame({'date':['2017-5-16',np.nan]})
df['trade_date_2']= pd.to_datetime(df['date']).dt.date
df['dow_2'] = df['trade_date_2'].apply(lambda x: x.weekday())
AttributeError: 'float' object has no attribute 'weekday'
AttributeError: 'float' 对象没有属性 'weekday'
Comparing solutions:
比较解决方案：
df = pd.DataFrame({'date':['2017-5-16','2017-5-17']})
df = pd.concat([df]*10000).reset_index(drop=True)
def a(df):
df['trade_date_2']= pd.to_datetime(df['date']).dt.date
df['dow_2'] = df['trade_date_2'].apply(lambda x: x.weekday())
return df
def b(df):
df['date1'] = pd.to_datetime(df['date'])
df['trade_date_21']= df['date1'].dt.date
df['dow_21']=df['date1'].dt.dayofweek
return (df)
def c(df):
#dont write to column, but to helper series
dates = pd.to_datetime(df['date'])
df['trade_date_22']= dates.dt.date
df['dow_22']= dates.dt.dayofweek
return (df)
In [186]: %timeit (a(df))
10 loops, best of 3: 101 ms per loop
In [187]: %timeit (b(df))
10 loops, best of 3: 90.8 ms per loop
In [188]: %timeit (c(df))
10 loops, best of 3: 91.9 ms per loop
