What is a good practice to check if an environmental variable exists or not

Question

I want to check my environment for the existence of a variable  say  FOO   in Python  For this purpose  I am using the os standard library  After reading the library s documentation  I have figured out 2 ways to achieve my goal   Method 1     if  FOO  in os environ      pass   Method 2   if os getenv  FOO   is not None      pass   I would like to know which method  if either  is a good preferred conditional and why

User · Answer

In case you want to check if multiple env variables are not set  you can do the following   import os  MANDATORY ENV VARS     FOO    BAR    for var in MANDATORY ENV VARS      if var not in os environ          raise EnvironmentError  Failed because    is not set   format var

User · Answer

There is a case for either solution  depending on what you want to do conditional on the existence of the environment variable   Case 1  When you want to take different actions purely based on the existence of the environment variable  without caring for its value  the first solution is the best practice  It succinctly describes what you test for  is  FOO  in the list of environment variables   if  KITTEN ALLERGY  in os environ      buy puppy   else      buy kitten     Case 2  When you want to set a default value if the value is not defined in the environment variables the second solution is actually useful  though not in the form you wrote it   server   os getenv  MY CAT STREAMS    youtube com     or perhaps  server   os environ get  MY CAT STREAMS    youtube com     Note that if you have several options for your application you might want to look into ChainMap  which allows to merge multiple dicts based on keys  There is an example of this in the ChainMap documentation         combined   ChainMap command line args  os environ  defaults

User · Answer

I d recommend the following solution  It prints the env vars you didn t include  which lets you add them all at once  If you go for the for loop  you re going to have to rerun the program to see each missing var  from os import environ  REQUIRED ENV VARS     quot A quot    quot B quot    quot C quot    quot D quot   diff   REQUIRED ENV VARS difference environ  if len diff   gt  0      raise EnvironmentError f Failed because  diff  are not set

User · Answer

My comment might not be relevant to the tags given  However  I was lead to this page from my search   I was looking for similar check in R and I came up the following with the help of  hugovdbeg post  I hope it would be helpful for someone who is looking for similar solution in R   USERNAME   in  names Sys getenv

User · Answer

To be on the safe side use  os getenv  FOO   or  bar    A corner case with the above answers is when the environment variable is set but is empty  For this special case you get  print os getenv  FOO    bar      prints new line - though you expected  bar    or   if  FOO  in os environ      print  FOO is here     prints FOO is here - however its not   To avoid this just use or  os getenv  FOO   or  bar    Then you get  print os getenv  FOO   or  bar     bar   When do we have empty environment variables   You forgot to set the value in the  env file     env FOO    or exported as    export FOO    or forgot to set it in settings py     settings py os environ  FOO            Update  if in doubt  check out these one-liners   gt  gt  gt  import os  os environ  FOO         print os getenv  FOO    bar       FOO  python -c  import os  print os getenv  FOO    bar

User · Answer

Use the first  it directly tries to check if something is defined in environ  Though the second form works equally well  it s lacking semantically since you get a value back if it exists and only use it for a comparison    You re trying to see if something is present in environ  why would you get just to compare it and then toss it away     That s exactly what getenv does      Get an environment variable  return None if it doesn t exist  The   optional second argument can specify an alternate default     this also means your check could just be if getenv  FOO     you don t want to get it  you want to check for it s existence    Either way  getenv is just a wrapper around environ get but you don t see people checking for membership in mappings with   from os import environ if environ get  Foo   is not None    To summarize  use    if  FOO  in os environ      pass   if you just want to check for existence  while  use getenv  FOO   if you actually want to do something with the value you might get

[python] What is a good practice to check if an environmental variable exists or not?

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