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How to fix date formatting using python3

I have data with the date format as follows:
date_format = 190410
year = 19
month = 04
date = 10
I want to change the date format, to be like this:
date_format = 10-04-2019
How do I solve this problem?

>>> import datetime
>>> date = 190410
>>> datetime.datetime.strptime(str(date), "%y%m%d").strftime("%d-%m-%Y")
'10-04-2019'
datetime.strptime() takes a data string and a format, and turns that into datetime object, and datetime objects have a method called strftime that turns datetime objects to string with given format. You can look what %y %m %d %Y are from here.

This is what you want(Notice that you have to change your format)
import datetime
date_format = '2019-04-10'
date_time_obj = datetime.datetime.strptime(date_format, '%Y-%m-%d')
print(date_time_obj)
Here is an other example
import datetime
date_time_str = '2018-06-29 08:15:27.243860'
date_time_obj = datetime.datetime.strptime(date_time_str, '%Y-%m-%d %H:%M:%S.%f')
print('Date:', date_time_obj.date())
print('Time:', date_time_obj.time())
print('Date-time:', date_time_obj)
You can also do this
from datetime import datetime, timedelta
s = "20120213"
# you could also import date instead of datetime and use that.
date = datetime(year=int(s[0:4]), month=int(s[4:6]), day=int(s[6:8]))
print(date)
There are many ways to achieve what you want.

Removing milliseconds from datetime object in Python

I am trying to remove the milliseconds(28109) from this string 2017-09-12 22:33:55.28109 in Python.
code:
import datetime as dt
from datetime import date,datetime
created_date = datetime.fromtimestamp(ctime)
d=datetime.strptime(created_date, "%Y-%m-%d %H:%M:%S.%fZ")
created_date = datetime.strftime(d, "%m/%d/%Y %I:%M:%S %p")
print(created_date)
Error:
`d=datetime.strptime(created_date, "%Y-%m-%d %H:%M:%S.%fZ")`
TypeError: must be str, not datetime.datetime

You already have a datetime object, you do not need to parse it again. The datetime.fromtimestamp() call was enough.
Remove the datetime.strptime() line.
created_date = datetime.fromtimestamp(ctime)
created_date = created_date.strftime("%m/%d/%Y %I:%M:%S %p")
print(created_date)
I also changed your strftime() call, it is a method, you just call it on the datetime object you have.
I suspect that you printed the return value of the datetime.fromtimestamp() call, and got confused. The str() conversion of a datetime() instance formats the value as a ISO 8601 string. Note that even if you did have a string, you used the wrong format (there is no timezone in that string, so %Z does not apply).
If you needed a datetime object, rather than a formatted string, you could also just have converted your timestamp to an integer; the microseconds are captured in the decimal portion of the timestamp:
>>> ctime = 1505252035.28109
>>> datetime.fromtimestamp(ctime)
datetime.datetime(2017, 9, 12, 22, 33, 55, 281090)
>>> datetime.fromtimestamp(int(ctime))
datetime.datetime(2017, 9, 12, 22, 33, 55)
>>> print(_)
2017-09-12 22:33:55

You can use time as well to achieve what you want.
import time
ctime = "2017-09-12 22:33:55.28109"
x = time.strptime(ctime.split('.')[0],'%Y-%m-%d %H:%M:%S')
x = time.strftime('%m/%d/%Y %I:%M:%S %p', x)
print (x)
'09/12/2017 10:33:55 PM'

Convert String to Python datetime Object without Zero Padding

I'm using python 3.5.
I have a string formatted as mm/dd/yyyy H:MM:SS AM/PM that I would like as a python datetime object.
Here is what I've tried.
date = "09/10/2015 6:17:09 PM"
date_obj = datetime.datetime.strptime(date, '%d/%m/%Y %I:%M:%S %p')
But this gets an error because the hour is not zero padded. The formatting was done per the table on the
datetime documentation, which does not allow the hour to have one digit.
I've tried splitting the date up, adding a zero and then reassembling the string back together, while this works, this seems less robust/ideal.
date = "09/10/2015 6:17:09 PM"
date = date.split()
date = date[0] + " 0" + date[1] + " " + date[2]
Any recommendation on how to get the datetime object directly, or a better method for padding the hour would be helpful.
Thank you.

There is nothing wrong with this code:
>>> date = "09/10/2015 6:17:09 PM"
>>> date_obj = datetime.datetime.strptime(date, '%m/%d/%Y %I:%M:%S %p')
>>> date_obj
datetime.datetime(2015, 9, 10, 18, 17, 9)
>>> print(date_obj)
2015-09-10 18:17:09
The individual attributes of the datetime object are integers, not strings, and the internal representation uses 24hr values for the hour.
Note that I have swapped the day and month in the format strings as you state that the input format is mm/dd/yyyy.
But it seems that you actually want it as a string with zero padded hour, so you can use datetime.strftime() like this:
>>> date_str = date_obj.strftime('%m/%d/%Y %I:%M:%S %p')
>>> print(date_str)
09/10/2015 06:17:09 PM
# or, if you actually want the output format as %d/%m/%Y....
>>> print(date_obj.strftime('%d/%m/%Y %I:%M:%S %p'))
10/09/2015 06:17:09 PM

how to convert a datetime string back to datetime object?

I am storing a datetime string in a database. Now I face a problem. When I fetch the string from the database, I need to convert it back to a datetime object...
Any easy way to do that?
The string of datetime looks like:
2010-11-13 10:33:54.227806

You want datetime.strptime(date_string, format).
from datetime import datetime
datetime.strptime("2010-11-13 10:33:54.227806", "%Y-%m-%d %H:%M:%S.%f")
For details on the format string, see http://docs.python.org/library/datetime.html#strftime-and-strptime-behavior

I sugggest you install python-dateutilt:
from dateutil import parser
d = parser.parse(yourstring)
This library gets a datetime object from your date-string in a 'smart' way...

# time tuple to datetime object
time_tuple = (2008, 11, 12, 13, 51, 18, 2, 317, 0)
dt_obj = datetime(*time_tuple[0:6])
print repr(dt_obj)
# date string to datetime object
date_str = "2008-11-10 17:53:59"
dt_obj = datetime.strptime(date_str, "%Y-%m-%d %H:%M:%S")
print repr(dt_obj)
# timestamp to datetime object in local time
timestamp = 1226527167.595983
dt_obj = datetime.fromtimestamp(timestamp)
print repr(dt_obj)

Python Datetime : use strftime() with a timezone-aware date

Suppose I have date d like this :
>>> d
datetime(2009, 4, 19, 21, 12, tzinfo=tzoffset(None, -7200))
As you can see, it is "timezone aware", there is an offset of 2 Hour, utctime is
>>> d.utctimetuple()
time.struct_time(tm_year=2009, tm_mon=4, tm_mday=19,
tm_hour=23, tm_min=12, tm_sec=0,
tm_wday=6, tm_yday=109, tm_isdst=0)
So, real UTC date is 19th March 2009 23:12:00, right ?
Now I need to format my date in string, I use
>>> d.strftime('%Y-%m-%d %H:%M:%S.%f')
'2009-04-19 21:12:00.000000'
Which doesn't seems to take this offset into account. How to fix that ?

In addition to what #Slam has already answered:
If you want to output the UTC time without any offset, you can do
from datetime import timezone, datetime, timedelta
d = datetime(2009, 4, 19, 21, 12, tzinfo=timezone(timedelta(hours=-2)))
d.astimezone(timezone.utc).strftime('%Y-%m-%d %H:%M:%S.%f')
See datetime.astimezone in the Python docs.

The reason is python actually formatting your datetime object, not some "UTC at this point of time"
To show timezone in formatting, use %z or %Z.
Look for strf docs for details

This will convert your local time to UTC and print it:
import datetime, pytz
from dateutil.tz.tz import tzoffset
loc = datetime.datetime(2009, 4, 19, 21, 12, tzinfo=tzoffset(None, -7200))
print(loc.astimezone(pytz.utc).strftime('%Y-%m-%d %H:%M:%S.%f') )
(http://pytz.sourceforge.net/)

I couldn't import timezone module (and hadn't much time to know why)
so I set TZ environment variable which override the /etc/localtime information
>>> import os
>>> import datetime
>>> print datetime.datetime.now().strftime('%Y-%m-%d %H:%M')
2019-05-17 11:26
>>> os.environ["TZ"] = "UTC"
>>> print datetime.datetime.now().strftime('%Y-%m-%d %H:%M')
2019-05-17 09:26