dask dataframe how to convert column to to_datetime

问题

I am trying to convert one column of my dataframe to datetime. Following the discussion here https://github.com/dask/dask/issues/863 I tried the following code:

import dask.dataframe as dd
df['time'].map_partitions(pd.to_datetime, columns='time').compute()

But I am getting the following error message

ValueError: Metadata inference failed, please provide `meta` keyword

What exactly should I put under meta? should I put a dictionary of ALL the columns in df or only of the 'time' column? and what type should I put? I have tried dtype and datetime64 but none of them work so far.

Thank you and I appreciate your guidance,

Update

I will include here the new error messages:

1) Using Timestamp

df['trd_exctn_dt'].map_partitions(pd.Timestamp).compute()

TypeError: Cannot convert input to Timestamp

2) Using datetime and meta

meta = ('time', pd.Timestamp)
df['time'].map_partitions(pd.to_datetime,meta=meta).compute()
TypeError: to_datetime() got an unexpected keyword argument 'meta'

3) Just using date time: gets stuck at 2%

    In [14]: df['trd_exctn_dt'].map_partitions(pd.to_datetime).compute()
[                                        ] | 2% Completed |  2min 20.3s

Also, I would like to be able to specify the format in the date, as i would do in pandas:

pd.to_datetime(df['time'], format = '%m%d%Y'

Update 2

After updating to Dask 0.11, I no longer have problems with the meta keyword. Still, I can't get it past 2% on a 2GB dataframe.

df['trd_exctn_dt'].map_partitions(pd.to_datetime, meta=meta).compute()
    [                                        ] | 2% Completed |  30min 45.7s

Update 3

worked better this way:

def parse_dates(df):
  return pd.to_datetime(df['time'], format = '%m/%d/%Y')

df.map_partitions(parse_dates, meta=meta)

I'm not sure whether it's the right approach or not

回答1:

Use `astype`

You can use the astype method to convert the dtype of a series to a NumPy dtype

df.time.astype('M8[us]')

There is probably a way to specify a Pandas style dtype as well (edits welcome)

Use map_partitions and meta

When using black-box methods like map_partitions, dask.dataframe needs to know the type and names of the output. There are a few ways to do this listed in the docstring for map_partitions.

You can supply an empty Pandas object with the right dtype and name

meta = pd.Series([], name='time', dtype=pd.Timestamp)

Or you can provide a tuple of (name, dtype) for a Series or a dict for a DataFrame

meta = ('time', pd.Timestamp)

Then everything should be fine

df.time.map_partitions(pd.to_datetime, meta=meta)

If you were calling map_partitions on df instead then you would need to provide the dtypes for everything. That isn't the case in your example though.

回答2:

Dask also come with to_timedelta so this should work as well.

df['time']=dd.to_datetime(df.time,unit='ns')

The values unit takes is the same as pd.to_timedelta in pandas. This can be found here.

回答3:

I'm not sure if it this is the right approach, but mapping the column worked for me:

df['time'] = df['time'].map(lambda x: pd.to_datetime(x, errors='coerce'))

回答4:

This worked for me

ddf["Date"] = ddf["Date"].map_partitions(pd.to_datetime,format='%d/%m/%Y',meta = ('datetime64[ns]'))

回答5:

If the datetime is in a non ISO format then map_partition yields better results:

import dask
import pandas as pd
from dask.distributed import Client
client = Client()

ddf = dask.datasets.timeseries()
ddf = ddf.assign(datetime=ddf.index.astype(object))
ddf = (ddf.assign(datetime_nonISO = ddf['datetime'].astype(str).str.split(' ')
                                 .apply(lambda x: x[1]+' '+x[0], meta=('object'))) 

%%timeit
ddf.datetime = ddf.datetime.astype('M8[s]')
ddf.compute()

11.3 s ± 719 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

ddf = dask.datasets.timeseries()
ddf = ddf.assign(datetime=ddf.index.astype(object))
ddf = (ddf.assign(datetime_nonISO = ddf['datetime'].astype(str).str.split(' ')
                                 .apply(lambda x: x[1]+' '+x[0], meta=('object'))) 


%%timeit
ddf.datetime_nonISO = (ddf.datetime_nonISO.map_partitions(pd.to_datetime
                       ,  format='%H:%M:%S %Y-%m-%d', meta=('datetime64[s]')))
ddf.compute()

8.78 s ± 599 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

ddf = dask.datasets.timeseries()
ddf = ddf.assign(datetime=ddf.index.astype(object))
ddf = (ddf.assign(datetime_nonISO = ddf['datetime'].astype(str).str.split(' ')
                                 .apply(lambda x: x[1]+' '+x[0], meta=('object'))) 

%%timeit
ddf.datetime_nonISO = ddf.datetime_nonISO.astype('M8[s]')
ddf.compute()

1min 8s ± 3.65 s per loop (mean ± std. dev. of 7 runs, 1 loop each)

来源：https://stackoverflow.com/questions/39584118/dask-dataframe-how-to-convert-column-to-to-datetime

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python

pandas

dask