Coordinated Universal Time(UTC):

It is the primary time standard by which the world regulates clocks and time. To get current UTC time in Python, we can use datetime module.

In [5]: import datetime

In [6]: datetime.datetime.now(datetime.timezone.utc)
Out[6]: datetime.datetime(2014, 11, 22, 14, 42, 21, 34435, tzinfo=datetime.timezone.utc)

Unix time / POSIX time / Epoch time:

It is a system for describing instants in time, defined as the number of seconds that have elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds. To get Unix timestamp, we can use time module.

In [8]: import time

In [9]: time.time()
Out[9]: 1416667432.5664258

In [10]: int(time.time())
Out[10]: 1416667766

With Pyhon 3.4, we can directly get timestamp from UTC.

In [13]: datetime.datetime.utcnow().timestamp()
Out[13]: 1416649608.58369

Conversions:

To convert Unix timestamp to UTC we can use utcfromtimestamp function.

In [21]: datetime.datetime.utcfromtimestamp(1416668401)
Out[21]: datetime.datetime(2014, 11, 22, 15, 0, 1)

To convert UTC time object to Unix time, we can use strftime function.

In [38]: dt = datetime.datetime.now()

In [39]: dt.strftime("%s")
Out[39]: '1416668938'

Alternatively, we can use calendar.timegen function.

In [46]: import calendar

In [47]: dt = datetime.datetime.utcnow()

In [48]: calendar.timegm(dt.utctimetuple())
Out[48]: 1416669150

When using iPython, users can import required modules to test them. After importing them, if they get modified either by user or some other process, users have to reload it for futher usage.

Depending on the Python version, appropriate reload function can reload modules.

# Python 2.x
In [15]: imp.reload(module)

# Python 3.0–3.3
In [15]: imp.reload(module)

# Python 3.4+
In [15]: importlib.reload(module)

Instead of manually reloading, ipython has autoreload extention which can auto reload modules. For that, load the extention and activate it.

In [15]: %load_ext autoreload

In [16]: %autoreload 2

This can be added to ipython config file so that autoreload gets activated, whenver it starts.

$ ipython profile create

This creates a default config file. Open config file which is present at ~/.ipython/profile_default/ipython_config.py and add these two lines to it.

c.InteractiveShellApp.extensions = ['autoreload']
c.InteractiveShellApp.exec_lines = ['%autoreload 2']

Note that it won't reloads c extensions automatically.

I was writing custom template tags for one of my Django package. I came across a situation where I had lists in string format. I need to convert them into lists.

temp_str = '[345, 3, 456, 45]'
required_obj = [345, 3, 456, 45]

There are two methods to get this job done.

ast.literal_eval:

This can be used to evaluate strings containing Python values from untrusted sources without parsing values. import ast ast.literal_eval(temp_str)

json.loads:

This is used to deserialize a string to python object using a conversion table.

import json
json.loads(temp_str)
These two functions come in handy whenever you want to convert a list of python objects to their correct types. For example, if you have list of python objects like this
obj_list =  ['hello', '3', '3.64', '-1']

You can convert them to their corresponding types using these functions.

def converter(l):
    for i in l:
        try:
            yield json.loads(i)
        except ValueError:
            yield i
def converter(l):
    for i in l:
        try:
            yield ast.literal_eval(i)
        except ValueError:
            yield i

They yield a objects of corresponding types new_obj_list = ['hello', 3, 3.64, -1]

Developers and hackers prefer using terminal and spend a lot of time on it. Instead of typing long commands over and over, they can be aliased to shortnames. The shell builtin alias allows users to set aliases.

One of the most used command while setting up development environment is pip install -r requirements.txt This can be aliased to pir.

alias pir='pip install -r requirements.txt

Now to install requirements, type pir and pressing enter. Here are some other aliases related to python which will be useful on a daily basis.

alias py='python'
alias ipy='ipython'
alias py3='python3'
alias ipy3='ipython3'

alias jn='jupyter notebook'

alias wo='workon'
alias pf='pip freeze | sort'
alias pi='pip install'
alias pun='pip uninstall'

alias dj="python manage.py"
alias drs="python manage.py runserver"
alias drp="python manage.py runserverplus"
alias drps="python manage.py runserverplus --print-sql"
alias dsh="python manage.py shell"
alias dsp="python manage.py shell_plus"
alias dsps="python manage.py shell_plus --print-sql"
alias dsm="python manage.py schemamigration"
alias dm="python manage.py migrate"
alias dmm="python manage.py makemigrations"
alias ddd="python manage.py dumpdata"
alias dld="python manage.py loaddata"
alias dt="python manage.py test"

Just add the above aliases to your ~/.bashrc or ~/.zshrc. That's it. Hpy alsng!