14 October 2018
This article will cover the most common problem people deal with when working with Pandas as it relates to time.
One of the most common things is to read timestamps into Pandas via CSV. If you just call read_csv, Pandas will read the data in as strings. We'll start with a super simple csv file
After calling read_csv, we end up with a Dataframe with an
object column. Which isn't really good for doing any date oriented analysis.
>>> df = pd.read_csv(data) >>> df Date 0 2018-01-01 >>> df.dtypes Date object dtype: object
We can use the parse_dates parameter to convince pandas to turn things into real
datetime types. parse_dates takes a list of columns (since you could want to parse multiple columns into
>>> df = pd.read_csv(data, parse_dates=['Date']) >>> df Date 0 2018-01-01 >>> df.dtypes Date datetime64[ns] dtype: object
Sometimes dates and times are split up into multiple columns. Pandas handles this just fine. Using this CSV
Date,Time 2018-01-01,10:30 2018-01-01,10:20
And the following code
>>> df = pd.read_csv(data, parse_dates=[['Date','Time']]) >>> df Date_Time 0 2018-01-01 10:30:00 1 2018-01-01 10:20:00
parse_dates is passed a nested list a more complex example might be the most straightforward way to illustrate why
birthday,last_contact_date,last_contact_time 1972-03-10,2018-01-01,10:30 1982-06-15,2018-01-01,10:20
>>> df = pd.read_csv(data, parse_dates=['birthday', ['last_contact_date','last_contact_time']]) >>> df last_contact_date_last_contact_time birthday 0 2018-01-01 10:30:00 1972-03-10 1 2018-01-01 10:20:00 1982-06-15
The top level list denotes each desired output datetime column, any nested fields refer to fields should be concatenated together.
By default Pandas uses
dateutil.parser.parse to parse strings into datetimes. There are times when you want to write your own.
From their documentation:
Function to use for converting a sequence of string columns to an array of datetime instances. The default uses dateutil.parser.parser to do the conversion. Pandas will try to call date_parser in three different ways, advancing to the next if an exception occurs: 1) Pass one or more arrays (as defined by parse_dates) as arguments; 2) concatenate (row-wise) the string values from the columns defined by parse_dates into a single array and pass that; and 3) call date_parser once for each row using one or more strings (corresponding to the columns defined by parse_dates) as arguments.
def myparser(date, time): data =  for d, t in zip(date, time): data.append(dt.datetime.strptime(d+t, "%Y-%m-%d%H:%M")) return data df = pd.read_csv(data, parse_dates=[['Date','Time']], date_parser=myparser)
That example function handles option 1 - where each array (in this case
Time are passed into the parser.
For this section, I've loaded data from the NYPD Motor Vehicle Collisions dataset
Once data has been loaded, filtering based on matching a given time or what is greater or less than other reference timestamps is one of the most common operations that people deal with.
For example if you want to subselect all data that occurs after
20170124 10:30, you can just do:
>>> ref = pd.Timestamp('20170124 10:30') >>> data[data.DATE_TIME > ref]
In interactive data analysis, the performance of such operations usually doesn't matter, however it can definitely matter when building applications where you perform lots of filtering.
In this case, dropping below Pandas into NumPy, and even converting from datetime64 to integers can help significantly. Note - converting to integers probably does
the wrong thing if you have any
NaT in your data.
>>> %timeit (data.DATE_TIME > ref) 3.67 ms ± 94.9 µs per loop (mean ± std. dev. of 7 runs, 100 loops each) >>> date_time = data.DATE_TIME.values >>> ref = np.datetime64(ref) >>> %timeit date_time > ref 1.57 ms ± 28.7 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each) >>> date_time = data.DATE_TIME.values.astype('int64') >>> ref = pd.Timestamp('20170124 10:30') >>> ref = ref.value >>> %timeit date_time > ref 764 µs ± 22.7 µs per loop (mean ± std. dev. of 7 runs, 1000 loops each)
If you have a Timestamp that is timezone Naive,
tz_localize will turn it into a timezone aware Timestamp.
tz_localize will also do the right thing for daylight savings time
>>> index = pd.DatetimeIndex([ ... dateutil.parser.parse('2002-10-27 04:00:00'), ... dateutil.parser.parse('2002-10-26 04:00:00') ... ]) >>> print(index) DatetimeIndex(['2002-10-27 04:00:00', '2002-10-26 04:00:00'], dtype='datetime64[ns]', freq=None) >>> index = index.tz_localize('US/Eastern') >>> print(index) DatetimeIndex(['2002-10-27 04:00:00-05:00', '2002-10-26 04:00:00-04:00'], dtype='datetime64[ns, US/Eastern]', freq=None)
Note that the first Timestamp has is UTC-5, and the second one is UTC-4 (because of dayling savings time)
If you want to go to another timezone, use
>>> index = index.tz_convert('US/Pacific') >>> print(index) DatetimeIndex(['2002-10-27 01:00:00-08:00', '2002-10-26 01:00:00-07:00'], dtype='datetime64[ns, US/Pacific]', freq=None)
Sometimes localizing will fail. You'll either get an
AmbiguousTimeError, or a
refers to cases where an hour is repeated due to daylight savings time.
NonExistentTimeError refers to cases where
an hour is skipped due to daylight savings time. There are currently options to either autocorrect
AmbiguousTimeError or return
but the corresponding settings for
NonExistentTimeError do not exists yet (looks like they'll be in soon)
No matter what timezone you're in, the underlying data (NumPy) is ALWAYS stored as nanoseconds since EPOCH in UTC.
If you're ever done groupby operations, resampling works in the same way, it just has nice conventions around time that are convenient. Here I'm aggregating all cycling injuries by day. I can also apply more complex functions. If I want to generate the same plot but Do it by borough:
One of the subtle things everyone who works with timestamps should be aware of is how pandas will label the result. From the documentation:
Which bin edge label to label bucket with. The default is ‘left’ for all frequency offsets except for ‘M’, ‘A’, ‘Q’, ‘BM’, ‘BA’, ‘BQ’, and ‘W’ which all have a default of ‘right’.
So longer horizon buckets are labeled at the end(right) of the bucket
>>> data.resample('M')['NUMBER OF CYCLIST INJURED'].sum().tail() DATE_TIME 2018-06-30 528 2018-07-31 561 2018-08-31 621 2018-09-30 533 2018-10-31 117 Freq: M, Name: NUMBER OF CYCLIST INJURED, dtype: int64
However shorter horizon buckets (including days) are labeled at the start(left) of the bucket
>>> data.resample('h')['NUMBER OF CYCLIST INJURED'].sum().tail() DATE_TIME 2018-10-08 19:00:00 0 2018-10-08 20:00:00 0 2018-10-08 21:00:00 2 2018-10-08 22:00:00 1 2018-10-08 23:00:00 1 Freq: H, Name: NUMBER OF CYCLIST INJURED, dtype: int64
This is really important to be aware of - because that means if you're doing any time based simulations on aggregations of data (I'm looking at you, FINANCE), and you're not careful about your labels, you could end up with lookahead bias in your simulation
In this example, I take the daily aggregations of cyclists injured in nyc, and I apply a 30 day moving sum.
I also demonstrate what happens if you resample it to 30 days. A few things become apparent
- the label is still on the left (even though it's a longer aggregation, it's based off of
1d so as a result the label is on the left)
- The data is sparser (since it ticks every 30 d)
The moving average case updates every day, aggregating the past 30 days. Looking at the tail of the data makes it more apparent.
>>> data.resample('d')['NUMBER OF CYCLIST INJURED'].sum().rolling(30).sum().tail() DATE_TIME 2018-10-04 510.0 2018-10-05 513.0 2018-10-06 504.0 2018-10-07 502.0 2018-10-08 501.0 Freq: D, Name: NUMBER OF CYCLIST INJURED, dtype: float64 >>> data.resample('30d')['NUMBER OF CYCLIST INJURED'].sum().tail() DATE_TIME 2018-05-31 00:05:00 519 2018-06-30 00:05:00 537 2018-07-30 00:05:00 596 2018-08-29 00:05:00 540 2018-09-28 00:05:00 176 Name: NUMBER OF CYCLIST INJURED, dtype: int64
As you can see when we resample by
30d we have a record every 30 days.
In the first case, we have a record every day (which is an aggregation of the past 30 days)
Pandas makes it super easy to do some crude seasonality analysis using the datetime accessors.
Any datetime column has a
dt attribute, which allows you to extract extra datetime oriented data.
if the index is a DatetimeIndex, you can access the same fields without the
for example, instead of resampling by
d, I could group by the date.
>>> data.groupby(data.index.date)['NUMBER OF CYCLIST INJURED'].sum() >>> data.groupby(data['DATE_TIME'].dt.date)['NUMBER OF CYCLIST INJURED'].sum().plot()
But this means you can get tons of insights by grouping by these fields:
Most injuries happen in the evening commute
Most injuries happen when it's warm (when most people cycle)
Less injuries occur on the weekend