Tag: timestamp

Python: Working with DateTimes

In this tutorial I will show you the different ways of working with dates and times in python. Which includes working with milliseconds. You should note this isn’t all available options just some that I have encountered over the years.

Install Python Packages:

Open cmd/terminal and if required navigate to your sites working folder. (note: if you are working in a virtual env you should ensure you source it first).

pip install python-dateutil

There are many different packages that we can use to work with date and times. You need to decide what is right for you.

dateutil:

The following will convert the date string you give it fast and easily. This gives you back the datetime object. Notice how we don’t need to pass it a date time format. To me this is very convenient.

from dateutil import parser

date_str = '2017-06-06'
date_time_str = '2017-06-07 12:34'
date_time_str_2 = '2017-06-07 12:34:46'
date_time_str_3 = '2017-06-07 12:34:42.234'

result = parser.parse(date_str)
print(result) #2017-06-06 00:00:00
result = parser.parse(date_time_str)
print(result) #2017-06-07 12:34:00
result = parser.parse(date_time_str_2)
print(result) #2017-06-07 12:34:46
result = parser.parse(date_time_str_3)
print(result) #2017-06-07 12:34:42.234000

datetime:

The following will convert the date string you give it fast and easily. This gives you back the datetime object. Notice how we need to pass the format of the datetime. If you don’t you will get an exception. This is a convenient way if you know the format before hand. But that might not always be the case.

import datetime

date_str = '2017-06-06'
date_time_str = '2017-06-07 12:34'
date_time_str_2 = '2017-06-07 12:34:46'
date_time_str_3 = '2017-06-07 12:34:42.234'

result = datetime.datetime.strptime(date_str, "%Y-%m-%d")
print(result) #2017-06-06 00:00:00
result = datetime.datetime.strptime(date_time_str, "%Y-%m-%d %H:%M")
print(result) #2017-06-07 12:34:00
result = datetime.datetime.strptime(date_time_str_2, "%Y-%m-%d %H:%M:%S")
print(result) #2017-06-07 12:34:46
result = datetime.datetime.strptime(date_time_str_3, "%Y-%m-%d %H:%M:%S.%f")
print(result) #2017-06-07 12:34:42.234000

The above all works however the following example will not. Why do you think this is?

import datetime

date_time_str = '2017-06-07 12:34:46'

try:
    datetime.datetime.strptime(date_time_str, "%Y-%m-%d %H:%M:%S")
except:
    pass #just for this example don't do this lol

The reason is because datetime expects the correct format to be supplied. We gave it hour minute second but not milliseconds. You will get the following exception (ValueError: unconverted data remains: .234)

Timestamps:

Sometimes we want to convert the date to unix (epoch) time or vise versa.

From Date:
from dateutil import parser
from datetime import timezone

date_time_str = '2017-06-07 17:34:42.234'
result = parser.parse(date_time_str)

timestamp = result.replace(tzinfo=timezone.utc).timestamp()
print(timestamp) #1496856882.234

This gives us the timestamp as a float as 1496856882.234.

From Timestamp:
from dateutil import parser
import datetime

timestamp = 1496856882.234

result = datetime.datetime.fromtimestamp(timestamp)
print(result) #2017-06-07 13:34:42.234000

result = datetime.datetime.utcfromtimestamp(timestamp)
print(result) #2017-06-07 17:34:42.234000

Get Date Parts:

If you want to get specific date parts such as the year, month, day, hour, etc.

import datetime
from dateutil import parser

result = parser.parse(date_time_str_3)
print(result) #2017-06-07 12:34:42.234000

year = result.year #2017
month = result.month #6
day = result.day #7
hour = result.hour #12
minute = result.minute #34
second = result.second #42
millisecond = result.microsecond #234000

Add To Date:

If you want to add time to a date.

import datetime
from dateutil import parser
from datetime import timezone, timedelta

date_time_str = '2017-06-07 17:34:42.234'
result = parser.parse(date_time_str)
print(result) #2017-06-07 17:34:42.234000

timestamp = result.replace(tzinfo=timezone.utc).timestamp()
print(timestamp) #1496856882.234

#Add 10 seconds to datetime
new_time = int((datetime.datetime.fromtimestamp(timestamp) + timedelta(milliseconds=10000)).timestamp() * 1000)
print(new_time) #1496856892234

As you can see you can 10 seconds has been added the datetime.

datetime strftime

from datetime import datetime

now = datetime.now()
datetime_str = now.strftime("%Y-%m-%d %H:%M:%S")
print(datetime_str)

datetime fromisoformat

from datetime import datetime

print(datetime.fromisoformat("2024-04-09 13:48:20"))

 

Postgres: Misc

Here are some misc things you can do in postgres 9.4.

Print to Console:

 raise notice 'VALUE: %', value;

Json Build Object:

 json_build_object('name', value, 'name2': value2)

Json Array:

 json_agg(data)

Extract Month:
You can also extract year, day, etc.

 extract(month from date_column)

Json Querying:
You can extract a field from a json field. You can do:

 rec->>'id'
rec#>'{field, sub_field,value}'
rec->'object'

Row To Json:

 SELECT row_to_json(t)
FROM mytable t

Update With FROM:

 UPDATE mytable l
SET old_val_id=sub.new_val_id
FROM (
     SELECT l2.id, s.id as new_val_id
     FROM mytable l2
          INNER JOIN mysupertable s ON s.id=l2.old_val_id
) sub
WHERE sub.id=l.id;

Inline Script:

 DO $do$
DECLARE id integer;
BEGIN      

END $do$ LANGUAGE plpgsql;

If:

 IF CONDITION THEN
END IF;

Let’s say you have a json field and in that field you have a field which tells you what key you should select for certain data. It could be customizable from the user entering data. To dynamically select that field you can do the below.

jsonField->((to_json((jsonField->'key'->'sub_key'))#>>'{}')::text)

Upsert:
Sometimes we want to perform update if the record exists or an insert if it doesn’t. Most of the time we write a function that deals with this on a record by record basis. But what if we want to do a batch update or insert. What do we do then. We perform an upsert. See below. We write the update and return the result in the remainder of the query  we check if upsert gave us anything. If it didn’t we perform the insert.

WITH upsert AS (UPDATE MyTable mt
            SET column_name = sub.new_value
            FROM (SELECT mot.id, mot.new_value 
                 FROM MyOtherTable mot
                 ) sub
            WHERE sub.id=mt.related_id 
RETURNING *) 
INSERT INTO MyTable (id, column_name, related_id) 
SELECT ?, ?, ? 
WHERE NOT EXISTS (SELECT * FROM upsert)

Regex Substring:
There are different ways of using regex. This is one way.

substring(column_name from '([0-9]{4}-[0-9]{2}-[0-9]{2})')

PGSQL Loop:
This is one way to loop using pgsql.

DO $do$
DECLARE rec RECORD;
BEGIN
	FOR rec IN SELECT * FROM MyTable
	LOOP
		--We can then use rec like rec.colum_name
	END LOOP;
END $do$ LANGUAGE plpgsql;

Milliseconds to timestamp:

This will return 2017-08-17 21:26:04

select to_timestamp(1503005165);