How can I access environment variables in Python?

How can I get the value of an environment variable in 开发者_JAVA技巧Python?

Environment variables are accessed through os.environ:

import os
print(os.environ['HOME'])

To see a list of all environment variables:

print(os.environ)

If a key is not present, attempting to access it will raise a KeyError. To avoid this:

# Returns `None` if the key doesn't exist
print(os.environ.get('KEY_THAT_MIGHT_EXIST'))

# Returns `default_value` if the key doesn't exist
print(os.environ.get('KEY_THAT_MIGHT_EXIST', default_value))

# Returns `default_value` if the key doesn't exist
print(os.getenv('KEY_THAT_MIGHT_EXIST', default_value))

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

'HOME' in os.environ

You can also use get() when printing the key; useful if you want to use a default.

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

where /home/username/ is the default

Actually it can be done this way:

import os

for item, value in os.environ.items():
    print('{}: {}'.format(item, value))

Or simply:

for i, j in os.environ.items():
    print(i, j)

For viewing the value in the parameter:

print(os.environ['HOME'])

Or:

print(os.environ.get('HOME'))

To set the value:

os.environ['HOME'] = '/new/value'

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)

You can access the environment variables using

import os
print os.environ

Try to see the content of the PYTHONPATH or PYTHONHOME environment variables. Maybe this will be helpful for your second question.

As for the environment variables:

import os
print os.environ["HOME"]

Import the os module:

import os

To get an environment variable:

os.environ.get('Env_var')

To set an environment variable:

# Set environment variables
os.environ['Env_var'] = 'Some Value'

import os
for a in os.environ:
    print('Var: ', a, 'Value: ', os.getenv(a))
print("all done")

That will print all of the environment variables along with their values.

If you are planning to use the code in a production web application code, using any web framework like Django and Flask, use projects like envparse. Using it, you can read the value as your defined type.

from envparse import env
# will read WHITE_LIST=hello,world,hi to white_list = ["hello", "world", "hi"]
white_list = env.list("WHITE_LIST", default=[])
# Perfect for reading boolean
DEBUG = env.bool("DEBUG", default=False)

NOTE: kennethreitz's autoenv is a recommended tool for making project-specific environment variables. For those who are using autoenv, please note to keep the .env file private (inaccessible to public).

There are also a number of great libraries. Envs, for example, will allow you to parse objects out of your environment variables, which is rad. For example:

from envs import env
env('SECRET_KEY') # 'your_secret_key_here'
env('SERVER_NAMES',var_type='list') #['your', 'list', 'here']

You can also try this:

First, install python-decouple

pip install python-decouple

Import it in your file

from decouple import config

Then get the environment variable

SECRET_KEY=config('SECRET_KEY')

Read more about the Python library here.

Edited - October 2021

Following @Peter's comment, here's how you can test it:

main.py

#!/usr/bin/env python


from os import environ

# Initialize variables
num_of_vars = 50
for i in range(1, num_of_vars):
    environ[f"_BENCHMARK_{i}"] = f"BENCHMARK VALUE {i}"  

def stopwatch(repeat=1, autorun=True):
    """
    Source: https://stackoverflow.com/a/68660080/5285732
    stopwatch decorator to calculate the total time of a function
    """
    import timeit
    import functools
    
    def outer_func(func):
        @functools.wraps(func)
        def time_func(*args, **kwargs):
            t1 = timeit.default_timer()
            for _ in range(repeat):
                r = func(*args, **kwargs)
            t2 = timeit.default_timer()
            print(f"Function={func.__name__}, Time={t2 - t1}")
            return r
        
        if autorun:
            try:
                time_func()
            except TypeError:
                raise Exception(f"{time_func.__name__}: autorun only works with no parameters, you may want to use @stopwatch(autorun=False)") from None
        
        return time_func
    
    if callable(repeat):
        func = repeat
        repeat = 1
        return outer_func(func)
    
    return outer_func

@stopwatch(repeat=10000)
def using_environ():
    for item in environ:
        pass

@stopwatch
def using_dict(repeat=10000):
    env_vars_dict = dict(environ)
    for item in env_vars_dict:
        pass

python "main.py"

# Output
Function=using_environ, Time=0.216224731
Function=using_dict, Time=0.00014206099999999888

If this is true ... It's 1500x faster to use a dict() instead of accessing environ directly.

A performance-driven approach - calling environ is expensive, so it's better to call it once and save it to a dictionary. Full example:

from os import environ


# Slower
print(environ["USER"], environ["NAME"])

# Faster
env_dict = dict(environ)
print(env_dict["USER"], env_dict["NAME"])

P.S- if you worry about exposing private environment variables, then sanitize env_dict after the assignment.

For Django, see Django-environ.

$ pip install django-environ

import environ

env = environ.Env(
    # set casting, default value
    DEBUG=(bool, False)
)
# reading .env file
environ.Env.read_env()

# False if not in os.environ
DEBUG = env('DEBUG')

# Raises Django's ImproperlyConfigured exception if SECRET_KEY not in os.environ
SECRET_KEY = env('SECRET_KEY')

You should first import os using

import os

and then actually print the environment variable value

print(os.environ['yourvariable'])

of course, replace yourvariable as the variable you want to access.

The tricky part of using nested for-loops in one-liners is that you have to use list comprehension. So in order to print all your environment variables, without having to import a foreign library, you can use:

python -c "import os;L=[f'{k}={v}' for k,v in os.environ.items()]; print('\n'.join(L))"