What is the quickest way to increment date string YYYY-MM-DD in Python?

Pure Python

You can use the datetime module, part of the standard library. There are 3 steps:

1. Convert string to datetime object via strptime.
2. Add a day via timedelta.
3. Convert resulting datetime object back to string via strftime.

Here's a demo:

from datetime import datetime, timedelta

x = '2017-05-15'
res = (datetime.strptime(x, '%Y-%m-%d') + timedelta(days=1)).strftime('%Y-%m-%d')

print(res)    
# 2017-05-16

Pandas

The equivalent steps can be performed using 3rd party Pandas:

x = '2017-05-15'

# choose some combination of below methods
res = (pd.Timestamp(x) + pd.DateOffset(days=1)).strftime('%Y-%m-%d')
res = (pd.to_datetime(x) + pd.Timedelta('1 day')).strftime('%Y-%m-%d')

print(res)
# 2017-05-16

Using pd.to_datetime, pd.TimeDelta and strftime:

fmt = '%Y-%m-%d'
(pd.to_datetime(<your series or column>, format=fmt) + pd.Timedelta('1 days')).dt.strftime(date_format=fmt)

Example

df = pd.DataFrame({'date': ['2017-04-02', '2017-04-23']})
fmt = '%Y-%m-%d'
>>> (pd.to_datetime(df.date, format=fmt) + pd.Timedelta('1 days')).dt.strftime(date_format=fmt)
0    2017-04-03
1    2017-04-24
Name: date, dtype: object