'<' not supported between instances of 'datetime.date' and 'str'

Question

I get a TypeError:

TypeError: '<' not supported between instances of 'datetime.date' and 'str'`

While running the following piece of code:

import requests
import re
import json
import pandas as pd

def retrieve_quotes_historical(stock_code):
    quotes = []
    url = 'https://finance.yahoo.com/quote/%s/history?p=%s' % (stock_code, stock_code)
    r = requests.get(url)
    m = re.findall('"HistoricalPriceStore":{"prices":(.*?), "isPending"', r.text)
    if m:
        quotes = json.loads(m[0])
        quotes = quotes[::-1]
    return  [item for item in quotes if not 'type' in item]

quotes = retrieve_quotes_historical('INTC')
df = pd.DataFrame(quotes)

s = pd.Series(pd.to_datetime(df.date, unit='s'))
df.date = s.dt.date
df = df.set_index('date')

This piece runs all smooth, but when I try to run this piece of code:

df['2017-07-07':'2017-07-10']

I get the TypeError.

How can I fix this?

Answer 1

The thing is you want to slice using Strings '2017-07-07' while your index is of type datetime.date. Your slices should be of this type too.

You can do this by defining your startdate and endate as follows:

import pandas as pd

startdate = pd.to_datetime("2017-7-7").date()
enddate = pd.to_datetime("2017-7-10").date()
df.loc[startdate:enddate]

startdate & enddate are now of type datetime.date and your slice will work:

            adjclose    close        high        low            open        volume
date
2017-07-07    33.205006    33.880001    34.119999    33.700001    33.700001    18304500
2017-07-10    32.979588    33.650002    33.740002    33.230000    33.250000    29918400

It is also possible to create datetime.date type without pandas:

import datetime

startdate = datetime.datetime.strptime('2017-07-07', "%Y-%m-%d").date()
enddate = datetime.datetime.strptime('2017-07-10', "%Y-%m-%d").date()

Answer 2

In addition to Paul's answer, a few things to note:

• pd.to_datetime(df['date'],unit='s') already returns a Series so you do not need to wrap it.

• besides, when parsing is successful the Series returned by pd.to_datetime has dtype datetime64[ns] (timezone-naïve) or datetime64[ns, tz] (timezone-aware). If parsing fails, it may still return a Series without error, of dtype O for "object" (at least in pandas 1.2.4), denoting falling back to Python's stdlib datetime.datetime.

• filtering using strings as in df['2017-07-07':'2017-07-10'] only works when the dtype of the index is datetime64[...], not when it is O (object

So with all of this, your example can be made to work by only changing the last lines:

df = pd.DataFrame(quotes)
s = pd.to_datetime(df['date'],unit='s')   # no need to wrap in Series
assert str(s.dtype) == 'datetime64[ns]'   # VERY IMPORTANT!!!!
df.index = s
print(df['2020-08-01':'2020-08-10'])    # it now works!

It yields:

                           date       open  ...    volume   adjclose
date                                        ...
2020-08-03 13:30:00  1596461400  48.270000  ...  31767100  47.050617
2020-08-04 13:30:00  1596547800  48.599998  ...  29045800  47.859154
2020-08-05 13:30:00  1596634200  49.720001  ...  29438600  47.654583
2020-08-06 13:30:00  1596720600  48.790001  ...  23795500  47.634968
2020-08-07 13:30:00  1596807000  48.529999  ...  36765200  47.105358
2020-08-10 13:30:00  1597066200  48.200001  ...  37442600  48.272457

Also finally note that if your datetime format somehow contains the time offset, there seem to be a mandatory utc=True argument to add (in Pandas 1.2.4) to pd.to_datetime, otherwise the returned dtype will be 'O' even if parsing is successful. I hope that this will improve in the future, as it is not intuitive at all.

See to_datetime documentation for details.