Which environment (as in environment variables) does PyCall use?

[goretkin]

I can set an environment variable as follows:

PyCall.PyDict(PyCall.pyimport("os").environ)["SOURCE_DATE_EPOCH"] = "0"

Using ENV in Julia did not work. I would like to understand which environment the python interpreter is in. As far as I understand, there isn't a separate process for the interpreter, so I am surprised there is a separate environment.

[stevengj]

See the Python docs for os.environ:

This mapping is captured the first time the os module is imported, typically during Python startup as part of processing site.py. Changes to the environment made after this time are not reflected in os.environ, except for changes made by modifying os.environ directly.

So, when PyCall is started, it gets a copy of Julia's environment. Subsequently modifying ENV (i.e. the actual environment) will not change os.environ (Python's copy).

[goretkin]

Ah, thanks for the explanation. So this is specific to how python handles environment variables. Unintuitive, but certainly documented.