Pythonic way to check if something exists?

Question

This is pretty basic but I was coding and started wondering if there was a pythonic way to check if something does not exist. Here's how I do it if its true:

var = 1
if var:
    print 'it exists'

but when I check if something does not exist, I often do something like this:

var = 2
if var:
    print 'it exists'
else:
    print 'nope it does not'

Seems like a waste if all I care about is knIs there a way to check if something does not exist without the else?

There's always ternary: http://stackoverflow.com/questions/394809/python-ternary-operator :) ... thought it might not be the most Pythonic way to write it out. — summea, Feb 22 '12 at 06:32
If `var` doesn't actually **exist**, then you are going to get an exception raised when you try to use it. That is outside of what `if`/`else` can handle. `if var` **assumes** that `var` exists, and **tests** if it is "true-ish" (becomes True rather than False if converted to boolean). — Karl Knechtel, Feb 22 '12 at 08:56
You are not checking the existence of a variable, but checking if it's value is `True`as a boolean context. — Mp0int, Jul 19 '13 at 10:02

wim · Accepted Answer · 2017-03-25T21:26:51.220

206

LBYL style, "look before you leap":

var_exists = 'var' in locals() or 'var' in globals()

EAFP style, "easier to ask forgiveness than permission":

try:
    var
except NameError:
    var_exists = False
else:
    var_exists = True

Prefer the second style (EAFP) when coding in Python, because it is generally more reliable.

edited Mar 25 '17 at 21:26

answered Feb 22 '12 at 06:40

wim

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Test if var does not exist: 'var' not in locals() and 'var' not in globals() – MyopicVisage Apr 26 '17 at 17:48

Paolo Bergantino · Answer 2 · 2012-02-22T06:44:04.357

42

I think you have to be careful with your terminology, whether something exists and something evaluates to False are two different things. Assuming you want the latter, you can simply do:

if not var:
   print 'var is False'

For the former, it would be the less elegant:

try:
   var
except NameError:
   print 'var not defined'

I am going to take a leap and venture, however, that whatever is making you want to check whether a variable is defined can probably be solved in a more elegant manner.

edited Feb 22 '12 at 06:44

answered Feb 22 '12 at 06:32

Paolo Bergantino

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sorry didn't mean false. meant if it does not exist. I use it more often in sql queries(I wrap an if statement in a select query and if no result comes back then I do some work), so not false looking for not existing. – Lostsoul Feb 22 '12 at 06:37
well `config = config()` and then `if not config:` gives me "local variable config referenced before assigment" :) – holms Feb 27 '13 at 10:32

score 6 · Answer 3 · answered Feb 22 '12 at 06:35

6

if not var:
    #Var is None/False/0/

if var:
    #Var is other then 'None/False/0'

in Python if varibale is having any value from None/False/0 then If var condition will fail...

and for other objects it will call __nonzero__ pythonic method which may return True or False depending on its functionality.

answered Feb 22 '12 at 06:35

avasal

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This will not handle the case where the variable was never defined, which is OP's question. – btown Nov 10 '13 at 21:36

score 6 · Answer 4 · edited Feb 07 '23 at 15:54

6

To check if a var has been defined:

var = 2

try: 
    var
except NameError:
    print("No var")

To check if it is None/False

if var is None

...or

if not var

edited Feb 07 '23 at 15:54

Choom

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answered Feb 22 '12 at 06:36

Calvin Froedge

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score 6 · Answer 5 · answered Feb 22 '12 at 06:39

If this is a dictionary, you can have

mydict['ggg'] = ''   // doesn't matter if it is empty value or not.
if mydict.has_key('ggg'):
   print "OH GEESH"

However, has_key() is completely removed from Python 3.x, therefore, the Python way is to use in

'ggg' in mydict     # this is it!
# True if it exists
# False if it doesn't

You can use in for tuple, list, and set as well.

Of course, if the variable hasn't been defined, you will have to raise an exception silently (just raise any exception... let it pass), if exception is not what you want to see (which is useful for many applications, you just need to log the exception.)

It is always safe to define a variable before you use it (you will run into "assignment before local reference" which means " var is not in the scope " in plain English). If you do something with query, the chance is, you will want to have a dictionary, and checking whether a key exists or not, use in .

Simplest of all the solutions – DrStrangepork Mar 01 '16 at 19:54 — DrStrangepork, Mar 01 '16 at 19:54
This doesn't work for a dictionary. – Jose4Linux Oct 21 '22 at 10:16 — Jose4Linux, Oct 21 '22 at 10:16

score 3 · Answer 6 · answered Feb 22 '12 at 06:33

3

What about the negation?

if not var:
    print 'nope it does not'

Though this is to see if the var is false / none and would blow up if var is not defined.

To see if var is defined:

try:
    var
except NameError:
    print 'not defined'

answered Feb 22 '12 at 06:33

manojlds

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Pythonic way to check if something exists?

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