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Difference between os.getenv and os.environ.get

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What is OS environ get?

environ in Python is a mapping object that represents the user's environmental variables. It returns a dictionary having user's environmental variable as key and their values as value. os. environ behaves like a python dictionary, so all the common dictionary operations like get and set can be performed.

What is the use of OS Getenv?

getenv() method in Python returns the value of the environment variable key if it exists otherwise returns the default value. default (optional) : string denoting the default value in case key does not exists. If omitted default is set to 'None'.

Where are OS environ variables stored?

Your environment variables are now stored in your . env file.

How do I set an environment variable in OS?

On the Windows taskbar, right-click the Windows icon and select System. In the Settings window, under Related Settings, click Advanced system settings. On the Advanced tab, click Environment Variables. Click New to create a new environment variable.


See this related thread. Basically, os.environ is found on import, and os.getenv is a wrapper to os.environ.get, at least in CPython.

EDIT: To respond to a comment, in CPython, os.getenv is basically a shortcut to os.environ.get ; since os.environ is loaded at import of os, and only then, the same holds for os.getenv.


One difference (observed in Python 2.7 and 3.8) between getenv() and environ[]:

  • os.getenv() does not raise an exception, but returns None
  • os.environ.get() similarly returns None
  • os.environ[] raises an exception if the environmental variable does not exist

In Python 2.7 with iPython:

>>> import os
>>> os.getenv??
Signature: os.getenv(key, default=None)
Source:
def getenv(key, default=None):
    """Get an environment variable, return None if it doesn't exist.
    The optional second argument can specify an alternate default."""
    return environ.get(key, default)
File:      ~/venv/lib/python2.7/os.py
Type:      function

So we can conclude os.getenv is just a simple wrapper around os.environ.get.


While there is no functional difference between os.environ.get and os.getenv, there is a massive difference between os.putenv and setting entries on os.environ. os.putenv is broken, so you should default to os.environ.get simply to avoid the way os.getenv encourages you to use os.putenv for symmetry.

os.putenv changes the actual OS-level environment variables, but in a way that doesn't show up through os.getenv, os.environ, or any other stdlib way of inspecting environment variables:

>>> import os
>>> os.environ['asdf'] = 'fdsa'
>>> os.environ['asdf']
'fdsa'
>>> os.putenv('aaaa', 'bbbb')
>>> os.getenv('aaaa')
>>> os.environ.get('aaaa')

You'd probably have to make a ctypes call to the C-level getenv to see the real environment variables after calling os.putenv. (Launching a shell subprocess and asking it for its environment variables might work too, if you're very careful about escaping and --norc/--noprofile/anything else you need to do to avoid startup configuration, but it seems a lot harder to get right.)