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How to run "source" command from python?

I need to set up the ROS2 Galactic environment by sourcing the following file through python: -
"source /opt/ros/galactic/setup.bash"
If I write the above line in terminal it will be sourced but I need to do this from python script.
I tried: -
import subprocess
subprocess.call("source /opt/ros/galactic/setup.bash", shell=True)
and
import os
os.system('source /opt/ros/galactic/setup.bash')
But none of them is sourcing the enviornment. I am working on Ubuntu 20.04, Python 3.8.10.

This will not impact your Python runtime environment. subprocess starts a shell wherein it runs your script (setup.bash) and then terminates.
Consider this:
import subprocess
import os
subprocess.run('export FOO=1', shell=True)
print(os.environ['FOO'])
This tries to set an environment variable in the sub-shell. That actually works but when run() returns, the shell no longer exists. Thus, when we try to access the environment variable we get KeyError

Import on shared object fails with ModuleNotFound only when script is executed from command line

ran into a weird problem where there is a shared-object import error only when I run the script from command line. It succeed if i run the script in the python console using exec(...)
I have a class that needs a shared object: foo.py:
import os
cur_dir = os.curdir()
os.chdir('/tmp/dir_with_shared_object/')
import shared_object_class
os.chdir(cur_dir)
class Useful:
... # use the shared object import
Then there is a script like this:
from foo import Useful
If I enter python console and run:
exec(open('script.py').read())
Everything works fine.
If I run this on command line:
python script.py
I will get
ModuleNotFoundError: No module named 'shared_object_class'
The python is the same. It is 3.7.3, GCC 7.3.0 Anaconda. Anyone knows what is causing this discrepancy in behavior for shared object import?

A standard way of importing from a custom directory would be to include it in the PYTHONPATH environmental variable, with export PYTHONPATH=/tmp/dir_with_shared_object/.
update
It could also be done dynamically with
import sys
p = '/tmp/dir_with_shared_object/'
sys.path.insert(0, p)
PS
I think I have an explanation for why OP's original code didn't work. According to this python reference page, the import system searches, inter alia, in "[t]he directory containing the input script (or the current directory when no file is specified)." So the behavior in the REPL loop is different from how it is when running a script. Apparently the current directory is evaluated each time an import statement is encountered, while the directory containing the input script doesn't change.

Mac gcloud install ImportError: No module named __future__

When installing gcloud for mac I get this error when I run the install.sh command according to docs here:
Traceback (most recent call last):
File "/path_to_unzipped_file/google-cloud-sdk/bin/bootstrapping/install.py", line 8, in <module>
from __future__ import absolute_import
I poked through and echoed out some stuff in the install shell script. It is setting the environment variables correctly (pointing to my default python installation, pointing to the correct location of the gcloud SDK).
If I just enter the python interpreter (using the same default python that the install script points to when running install.py) I can import the module just fine:
>>> from __future__ import absolute_import
>>>
Only other information worth noting is my default python setup is a virtual environment that I create from python 2.7.15 installed through brew. The virtual environment python bin is first in my PATH so python and python2 and python2.7 all invoke the correct binary. I've had no other issues installing packages on this setup so far.
If I echo the final line of the install.sh script that calls the install.py script it shows /path_to_virtualenv/bin/python -S /path_to_unzipped_file/google-cloud-sdk/bin/bootstrapping/install.py which is the correct python. Or am I missing something?

The script uses the -S command-line switch, which disables loading the site module on start-up.
However, it is a custom dedicated site module installed in a virtualenv that makes a virtualenv work. As such, the -S switch and virtualenvs are incompatible, with -S set fundamental imports such as from __future__ break down entirely.
You can either remove the -S switch from the install.bat command or use a wrapper script to strip it from the command line as you call your real virtualenv Python.

I had the error below when trying to run gcloud commands.
File "/usr/local/Caskroom/google-cloud-sdk/latest/google-cloud-sdk/lib/gcloud.py", line 20, in <module>
from __future__ import absolute_import
ImportError: No module named __future__
If you have your virtualenv sourced automatically you can specify the environment variable CLOUDSDK_PYTHON i.e. set -x CLOUDSDK_PYTHON /usr/bin/python to not use the virtualenv python.

In google-cloud-sdk/install.sh go to last line, remove variable $CLOUDSDK_PYTHON_ARGS as below.
"$CLOUDSDK_PYTHON" $CLOUDSDK_PYTHON_ARGS "${CLOUDSDK_ROOT_DIR}/bin/bootstrapping/install.py" "$#"
"$CLOUDSDK_PYTHON" "${CLOUDSDK_ROOT_DIR}/bin/bootstrapping/install.py" "$#"

Launch Python Shell with Predefined Imports and Settings

I am trying to set up a shell-launch script like this:
./practice.py
I intend to debug commands using various modules which can be tedious to import, set up oauth settings, etc.
How do I set example practice.py to launch with this sort of preset:
import module_x
import module_y
from module_z import something
a = something.settings(var_1='123', var_1='456')
>> #start doing stuff

You can use the PYTHONSTARTUP environmental variable. Quoting the documentation:
PYTHONSTARTUP
If this is the name of a readable file, the Python commands in
that file are executed before the first prompt is displayed in
interactive mode. The file is executed in the same namespace where
interactive commands are executed so that objects defined or imported
in it can be used without qualification in the interactive session.
You can also change the prompts sys.ps1 and sys.ps2 and the hook
sys.__interactivehook__ in this file.
How you set environmental variables depends on the OS.

Put a shebang at first line with interactive mode switch:
#!/usr/bin/env python -i
import module_x
import module_y
from module_z import something
a = something.settings(var_1='123', var_1='456')
Then make sure it is executable:
chmod +x practice.py
Then you can run it like this:
./practice.py

python (and the more comfortable ipython) support -i somefile.py as option to execute the contents of a file before opening the interactive prompt.
on Linux/Unix/OS X you could simply set that as the interpreter of your script like this
#! /usr/bin/python -i
import module_x
# other preloaded settings
On Windows you need to create a batch file or similar to wrap it.

Running profile startup files in an embedded IPython instance

I want to use an embedded IPython shell with a user_ns dictionary and a my profile configuration (ipython_config.py and the startup files). The purpose is to run a Django shell with models imported on startup. django-extensions implements a command called shell_plus that does this:
https://github.com/django-extensions/django-extensions/blob/master/django_extensions/management/commands/shell_plus.py
from IPython import embed
embed(user_ns=imported_objects)
The problem is that this does not load my startup files. embed() calls load_default_config() which I figure loads ipython_config.py.
How do I make the embedded IPython instance run my profile startup files?

I used the following workaround to run my own IPython startup script but still take advantage of shell_plus:
Create a file called shell_plus_startup.py in the same directory as manage.py. For example:
# File: shell_plus_startup.py
# Extra python code to run after shell_plus starts an embedded IPython shell.
# Run this file from IPython using '%run shell_plus_startup.py'
# Common imports
from datetime import date
# Common variables
tod = date.today()
Launch shell plus (which launches an embedded IPython shell).
python manage.py shell_plus
Manually run the startup script.
In [1]: %run shell_plus_startup.py
Then you can use variables you've defined, modules you've imported, etc.
In [2]: tod
Out[2]: datetime.date(2012, 7, 14)
Also see this answer: scripting ipython through django's shell_plus

I found a way that works if you are using django-extensions-shell_plus. It is a bit hacky, but with this way your startup file is loaded fully automatically and you don't have to type any run-command at the beginning of your ipython-session.
Therefore I edited the file shells.py from the django_extensions dir, which is in my case located in /usr/local/lib/python2.7/dist-packages/django_extensions/management/shells.py. I added these lines inside the function import_objects(options, style):, so it imports the content of the file startup.py defined by the environment param PYTHONSTARTUP.
def import_objects(options, style):
# (...)
import os, sys, pkgutil
if 'PYTHONSTARTUP' in os.environ:
try:
sys.path.append(os.environ['PYTHONSTARTUP'])
import startup
content = [element for element in dir(startup) if not element.startswith('__')]
for element in content:
imported_objects[element] = getattr(startup, element)
except Exception, ex:
sys.exit("Could not import startup module content, Error:\n%s" % ex)
Now when I launch the shell_plus-shell, I give it the environment variable to my startup python script. My bash script to launch the shell with everything in place looks like this:
#!/bin/bash
export PYTHONSTARTUP=/home/ifischer/src/myproject/startup.py # tells shell_plus to load this file
python /home/ifischer/src/myproject/manage.py shell_plus --ipython
Now I have access to all methods and variables defined in startup.py from the beginning of the ipython session.
So you can reuse that and have custom startup files for every project, pre-loading different aspects.
Maybe there is a cleaner way where to include the lines I added to the shells.py? But this approach works fine for me for now.

It does automatically load your ipython configuration starting from django-extensions==1.5.6. You can also pass additional arguments to ipython via IPYTHON_ARGUMENTS. Docs:
http://django-extensions.readthedocs.org/en/latest/shell_plus.html#configuration

Another way is using a class that derives from InteractiveShellEmbed and InteractiveShellApp. Sample, incomplete code:
from IPython.terminal.embed import InteractiveShellEmbed
from IPython.terminal.ipapp import InteractiveShellApp
class ISE(InteractiveShellEmbed, InteractiveShellApp):
def init_shell(self):
self.shell = self.instance()
self.extra_args = []
ise = ISE()
ise.init_path()
ise.init_shell()
ise.init_code()
ise.shell()