How can I determine if a python script is executed from crontab?

Check its PPID - the ID of its parent process. Compare that to the cron pid; If they are the same, it was invoked by the crontab.

This can be done by:

$ sudo ps -Af | grep cron | grep -v grep
root  6363  1  0 10:17 ?  00:00:00 /usr/sbin/cron

The PID of the cron process in this example is 6363. It is worth mentioning that the PPID of cron is 1 - the init process.

Now find out what is the PID of your python program:

$  sudo ps -Af | grep SorinSbarnea.py
adam  12992  6363  1 12:24 pts/2  00:04:21 /usr/bin/python SorinSbarnea.py

Its PID is 12992 and PPID is 6363, so it was indeed invoked by the cron process.

EDIT:

The cron process might not invoke your process directly. Hence, you'll have to traverse the PPIDs from your process upwards, till reaching PPID=1 or PPID=/usr/sbin/cron's PID. This can easily be done using a small shell or python script; just parse the output of ps:

$ cat /proc/12992/status
....
Pid:    12992
PPid:   7238
Uid:    1000    1000    1000    1000
Gid:    1000    1000    1000    1000
...

The next step would be checkig /proc/7238, and so forth. Again, This is really easy to implement using shell or python script.

Thanks, @digitalarbeiter and @Noufal Ibrahim for pointing it out in your comments.

Set an environment variable at the cron command invocation. That works even within a module, as you can just check os.getenv().

The following shows how to detect whether the program has a terminal and whether it has a display. As a cron job, both will be returned as None.

#!/usr/bin/python

import os

term = os.getenv( "TERM" )
print( "term %s"%term )

display = os.getenv( "DISPLAY" )
print( "display %s"%display )

The output when run from a desktop terminal session will be similar to the following:

term xterm-256color
display :0

And when it is run as a cron job, the output will be:

term None
display None

You can test for the presence of either of these, like this

if term is None:
    print( "no terminal" )

if display is None:
    print( "no display" )

For one further example, the following selects an appropriate matplotlib backend when run from a cron job:

if os.getenv( "DISPLAY" ) is None:
    print( "Using Agg" )
    import matplotlib
    matplotlib.use('Agg')

An easier workaround would be to pass a flag to the script only from the crontab, like --crontab, and then just check for that flag.

If you want to detect this from an imported module, I would have the main program set a global variable in the module, which would output different things depending on the value of this global variable (and have the main program decide how to set the variable through a flag that you would use in your crontab). This is quite robust (comparing to studying PPIDs).