[python] How to set environment variables in Python?

I need to set some environment variables in the Python script and I want all the other scripts that are called from Python to see the environment variables' set.

If I do,

os.environ["DEBUSSY"] = 1

it complains saying that 1 has to be a string.

I also want to know how to read the environment variables in Python (in the latter part of the script) once I set it.

Use setdefault function to set a new variable if the variable does not exist in the environment.

make sure you set the environment variable as a string, not int. Otherwise will throw TypeError.

import os

if not os.environ.get("DEBUSSY"):
    os.environ.setdefault("DEBUSSY","1")
else:
     os.environ["DEBUSSY"] = "1"

print(os.environ["DEBUSSY"])

os.environ behaves like a python dictionary, so all the common dictionary operations can be performed. In addition to the get and set operations mentioned in the other answers, we can also simply check if a key exists. The keys and values should be stored as strings.

Python 3

For python 3, dictionaries use the in keyword instead of has_key

>>> import os
>>> 'HOME' in os.environ  # Check an existing env. variable
True
...

Python 2

>>> import os
>>> os.environ.has_key('HOME')  # Check an existing env. variable
True
>>> os.environ.has_key('FOO')   # Check for a non existing variable
False
>>> os.environ['FOO'] = '1'     # Set a new env. variable (String value)
>>> os.environ.has_key('FOO')
True
>>> os.environ.get('FOO')       # Retrieve the value
'1'

There is one important thing to note about using os.environ:

Although child processes inherit the environment from the parent process, I had run into an issue recently and figured out, if you have other scripts updating the environment while your python script is running, calling os.environ again will not reflect the latest values.

Excerpt from the docs:

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.

os.environ.data which stores all the environment variables, is a dict object, which contains all the environment values:

>>> type(os.environ.data)  # changed to _data since v3.2 (refer comment below)
<type 'dict'>

I have been trying to add environment variables. My goal was to store some user information to system variables such that I can use those variables for future solutions, as an alternative to config files. However, the method described in the code below did not help me at all.

import os
os.environ["variable_1"] = "value_1"
os.environ["variable_2"] = "value_2"
# To Verify above code
os.environ.get("variable_1")
os.environ.get("variable_2")

This simple code block works well, however, these variables exist inside the respective processes such that you will not find them in the environment variables tab of windows system settings. Pretty much above code did not serve my purpose. This problem is discussed here: variable save problem

os.environ.putenv(key, value)

Another unsuccessful attempt. So, finally, I managed to save variable successfully inside the window environment register by mimicking the windows shell commands wrapped inside the system class of os package. The following code describes this successful attempt.

os.system("SETX {0} {1} /M".format(key, value))

I hope this will be helpful for some of you.

You can use the os.environ dictionary to access your environment variables.

Now, a problem I had is that if I tried to use os.system to run a batch file that sets your environment variables (using the SET command in a **.bat* file) it would not really set them for your python environment (but for the child process that is created with the os.system function). To actually get the variables set in the python environment, I use this script:

import re
import system
import os

def setEnvBat(batFilePath, verbose = False):
    SetEnvPattern = re.compile("set (\w+)(?:=)(.*)$", re.MULTILINE)
    SetEnvFile = open(batFilePath, "r")
    SetEnvText = SetEnvFile.read()
    SetEnvMatchList = re.findall(SetEnvPattern, SetEnvText)

    for SetEnvMatch in SetEnvMatchList:
        VarName=SetEnvMatch[0]
        VarValue=SetEnvMatch[1]
        if verbose:
            print "%s=%s"%(VarName,VarValue)
        os.environ[VarName]=VarValue

It should be noted that if you try to set the environment variable to a bash evaluation it won't store what you expect. Example:

from os import environ

environ["JAVA_HOME"] = "$(/usr/libexec/java_home)"

This won't evaluate it like it does in a shell, so instead of getting /Library/Java/JavaVirtualMachines/jdk1.8.0_144.jdk/Contents/Home as a path you will get the literal expression $(/usr/libexec/java_home).

Make sure to evaluate it before setting the environment variable, like so:

from os import environ
from subprocess import Popen, PIPE

bash_variable = "$(/usr/libexec/java_home)"
capture = Popen(f"echo {bash_variable}", stdout=PIPE, shell=True)
std_out, std_err = capture.communicate()
return_code = capture.returncode

if return_code == 0:
    evaluated_env = std_out.decode().strip()
    environ["JAVA_HOME"] = evaluated_env
else:
    print(f"Error: Unable to find environment variable {bash_variable}")

You should assign string value to environment variable.

os.environ["DEBUSSY"] = "1"

If you want to read or print the environment variable just use

print os.environ["DEBUSSY"]

This changes will be effective only for the current process where it was assigned, it will no change the value permanently. The child processes will automatically inherit the environment of the parent process.

if i do os.environ["DEBUSSY"] = 1, it complains saying that 1 has to be string.

Then do

os.environ["DEBUSSY"] = "1"

I also want to know how to read the environment variables in python(in the later part of the script) once i set it.

Just use os.environ["DEBUSSY"], as in

some_value = os.environ["DEBUSSY"]

Environment variables must be strings, so use

os.environ["DEBUSSY"] = "1"

to set the variable DEBUSSY to the string 1.

To access this variable later, simply use:

print(os.environ["DEBUSSY"])

Child processes automatically inherit the environment variables of the parent process -- no special action on your part is required.

What about os.environ["DEBUSSY"] = '1'? Environment variables are always strings.

to Set Variable:

item Assignment method using key:

import os    
os.environ['DEBUSSY'] = '1'  #Environ Variable must be string not Int

to get or to check whether its existed or not,

since os.environ is an instance you can try object way.

Method 1:

os.environ.get('DEBUSSY') # this is error free method if not will return None by default

will get '1' as return value

Method 2:

os.environ['DEBUSSY'] # will throw an key error if not found!

Method 3:

'DEBUSSY' in os.environ  # will return Boolean True/False

Method 4:

os.environ.has_key('DEBUSSY') #last 2 methods are Boolean Return so can use for conditional statements

You may need to consider some further aspects for code robustness;

when you're storing an integer-valued variable as an environment variable, try

os.environ['DEBUSSY'] = str(myintvariable)

then for retrieval, consider that to avoid errors, you should try

os.environ.get('DEBUSSY', 'Not Set')

possibly substitute '-1' for 'Not Set'

so, to put that all together

myintvariable = 1
os.environ['DEBUSSY'] = str(myintvariable)
strauss = int(os.environ.get('STRAUSS', '-1'))
# NB KeyError <=> strauss = os.environ['STRAUSS']
debussy = int(os.environ.get('DEBUSSY', '-1'))

print "%s %u, %s %u" % ('Strauss', strauss, 'Debussy', debussy)

When you play with environment variables (add/modify/remove variables), a good practice is to restore the previous state at function completion.

You may need something like the modified_environ context manager describe in this question to restore the environment variables.

Classic usage:

with modified_environ(DEBUSSY="1"):
    call_my_function()