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up vote 583 down vote favorite
109

I set an environment variable that I want to access in my Python application. How do I get this value?

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1  
I was trying to find how to access environment variables in Python, but instead of getting this question or Python manual, I got "The Other Questions Site" and I'm wondering how this could be corrected. – Juha Autero Feb 18 '11 at 18:38
up vote 920 down vote accepted

Environment variables are accessed through os.environ

import os
print os.environ['HOME']

Or you can see a list of all the environment variables using:

os.environ

As sometimes you might need to see a complete list!

# using get will return `None` if a key is not present rather than raise a `KeyError`
print os.environ.get('KEY_THAT_MIGHT_EXIST')

# os.getenv is equivalent, and can also give a default value instead of `None`
print os.getenv('KEY_THAT_MIGHT_EXIST', default_value)

Python default installation on Windows is C:\Python. If you want to find out while running python you can do:

import sys
print sys.prefix
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hello rod, thanks for your effective reply concerning 'default-installation'; effective in point of view to understand it quickly rather than go through the links. That’s really I appreciated :) but about my (1) question please look at the command and outputs snippet below: >>> import os >>> print os.environ['PYTHONPATH'] Traceback (most recent call last): File "<console>", line 1, in <module> File "C:\Python25\lib\os.py", line 435, in getitem return self.data[key.upper()] KeyError: 'PYTHONPATH' >>> print os.environ.get('PYTHONPATH') None >>> // PLZ to be continue...// – Amit Yadav Feb 5 '11 at 14:47
    
In a 1st way script is throwing Exception whereas with 2nd one giving None. So, is there any way to get it meaningful value or am I doing in a wrong way??? Amit. – Amit Yadav Feb 5 '11 at 14:49
2  
os.environ is a dictionary. Trying to access a key not present in the dictionary will throw a KeyError. The get method simply returns None when the key does not exists. Do you have PYTHONPATH set? Can you try with a variable such as PATH, that is guaranteed to exist? Does it return a meaningful value? – Rod Feb 5 '11 at 19:21
2  
PYTHONPATH is used to add new search path to Python (sys.path) from outside Python. Have a look at docs.python.org/using/cmdline.html#environment-variables – Rod Feb 7 '11 at 14:41
1  
Actually, os.environ, being a dictionary, may also return a value using its get method using os.environ.get("SOME_PARAMETER", "default_value") – sheba Dec 23 '15 at 16:11
up vote 63 down vote

To check if the key exists (returns True/False)

"HOME" in os.environ

or (removed from python 3.x)

os.environ.has_key("HOME")

You can also use get when printing the key, useful if you want to use a default. ( for python 2.7.3 )

print os.environ.get('HOME','/home/username/')

where /home/username/ is the default

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9  
You should use in instead of has_key. See stackoverflow.com/questions/1323410/has-key-or-in Also, the default kwarg name could be omitted for brevity (os.environ.get('HOME', '/home/username')). – Danilo Bargen Jul 12 '12 at 11:54
up vote 12 down vote

You can access to the environment variables using

import os
print os.environ

Try to see the content of PYTHONPATH or PYTHONHOME environment variables, maybe this will be helpful for your second question. However you should clarify it.

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up vote 12 down vote

The original question (first part) was "how to check environment variables in Python."

Here's how to check if $FOO is set: 

try:  
   os.environ["FOO"]
except KeyError: 
   print "Please set the environment variable FOO"
   sys.exit(1)
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26  
"FOO" in os.environ – erjoalgo Oct 17 '13 at 12:08
up vote 4 down vote

As for the environment variables:

import os
print os.environ["HOME"]

I'm afraid you'd have to flesh out your second point a little bit more before a decent answer is possible.

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