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Python's os.environ is surprisingly liberal in some ways

The way you access and modify Unix environment variables in Python programs is generally through os.environ; Python 3 being Python 3, sometimes you need os.environb. In Unix, what can go in the environment is somewhat fuzzy and while Python has some issues with character encodings, it's otherwise surprisingly liberal in a number of ways.

The first way that os.environ is liberal is that it allows environment variables to have blank values:

>>> os.environ["FRED"] = ""
>>> subprocess.run("printenv")

It's possible to do this with some Unix shells as well, but traditionally environment variables are generally assumed to have non-blank values. Quite a lot of code is likely to assume that a blank value is the same as the variable being unset, although in Python you can tell the difference since os.environ raises KeyError if the environment variable doesn't exist at all.

A bigger way that os.environ is liberal is that it will allow you to use non-traditional characters in the names of environment variables:

>>> os.environ["FRED/BAR"] = "Yes"
>>> subprocess.run("printenv")

On Unix, setting an environment variable uses setenv(), which generally only requires that you avoid '='. Python specifically checks for an '=' in your name so that it can generate a specific error, and otherwise passes things through.

Python itself doesn't particularly restricted environment variable names beyond that. As a result you can do all sorts of odd things with environment variable names, including putting spaces and Unicode into them (at least in a UTF-8 environment). Some or many of these environment variables won't be accessible to a shell program, but not everything that interprets the environment follows the shell's rules.

The case where this came up for me recently was in Dovecot post-login scripting, which in some cases can require you to create environment variables with '/' in their names. Typical shells disallow this, but I was quite happy to find that Python was perfectly willing to go ahead and everything worked fine.

python/OsEnvironLiberal written at 23:24:29; Add Comment

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