Add option to overwrite existing environment variables in read_env

[kutenai]

This PR addresses issue #103.
I've had this issue with the uwsgi touch-reload command. It won't pick up updates to .env files.

In some use cases, the .env file should be the master, so an overwrite=True option will resolve the issue in this case.

• All tests pass
• Should be no performance penalty for current use (other than an additional function call)
• Minor performance in overwrite use due to temp dict.

The temp dict was needed since there is no dict.set(k,v) method. The lambda can't do dict[k] = v.

[coveralls]

Coverage increased (+3.7%) to 89.376% when pulling 56772cb on sharpertool:develop into c262002 on joke2k:develop.

[coveralls]

Coverage increased (+3.7%) to 89.401% when pulling e979562 on sharpertool:develop into c262002 on joke2k:develop.

[BernardoSilva]

This would be very useful to have 👍

[JakeUrban]

@joke2k Can we get this merged?

[TmLev]

Any chance this will be merged soon?

[pymen]

I think the idea that it should not override original env variables [os.environ]

You can use the following helper to merge envs from files

def merge_files_env_vars(*files):
    """Merge env variables from files.
    order of files is important, bc next file can override overlapping values from previous files
    
    files are processed in reverse order bc environ uses setdefault, 
    so the latest file env can not be overwritten by previous
    """
    env = environ.Env()
    env.ENVIRON = {}
    for file_path in reversed(files):
        env.read_env(file_path)
    
    return env.ENVIRON

and later load them by

    env = environ.Env()
    env_variables = merge_files_env_vars(*env_files)
    env.read_env(**env_variables)

[sergeyklay]

PR was closed by mistake. Reopened it.

[sergeyklay]

This is implemented in develop branch. Thank you for the pathc, and for helping us make django-environ better. And I am sorry about the delay.