Determine if variable is defined in Python [duplicate]

Question

How do you know whether a variable has been set at a particular place in the code at runtime? This is not always obvious because (1) the variable could be conditionally set, and (2) the variable could be conditionally deleted. I'm looking for something like defined() in Perl or isset() in PHP or defined? in Ruby.

if condition:
    a = 42

# is "a" defined here?

if other_condition:
    del a

# is "a" defined here?

Artis

Duplicates: #843777, #750798, – Grodin

Please, please, please, do not "conditionally" set a variable. That's just dreadful, horrible design. Always, always, always provide for all logic paths. Please do not continue doing this. – Radii

+1 to S.Lott anwser. The fact that there is a way to do it doesn't mean you should use it in a fresh project. – Cordiecordier

@S.Lott: I have to disagree. When I use import to "source" a "config file" (i.e. a file that only has assignments in it), it may very well be that some variable has not been defined there. And import is sure cheaper/simpler than ConfigParser, so for me pragmatism wins over beauty here. – Restivo

@STATUS_ACCESS_DENIED: That's what defaults are appropriate. First set the defaults. Then import your configuration. Now all variables are defined, either as defaults or overrides in the configuration file. You can easily avoid conditionally setting a variable. – Radii

@S.Lott: What about in a memoizing method? You can return or set conditionally. It's a std in ruby, that way of thinking is why I am here. – Prebendary

dumb question here, I know I'm overthinking this... By "conditionally setting a variable", are you saying the existence of a variable shouldn't be based on a condition? – Distinctive

@dangel, yes. You may think "that variable will never be referenced if not condition", however it has been shown time and again (personal experience, at least) that programmers are imperfect and logic flows can in fact be different than expectations. This is especially true in spiral-development, where assumptions change and not all code is audited/refactored. It costs "almost nothing" to set the variable regardless of condition, even if only used when condition is met. – Rives

@Restivo The import machinery is heavy. And it is not intended to be used for "configuration files". If thats the only thing you want, you'd be better of using exec or eval. However, that would be very insecure in many ways, so I think its worth a simple parser. Anyway, the nearest equivalent to an isset() would be hasattr(). One could also use globals().get() which is capable of returning a default. – Gnosticism

Answer 1

try:
    thevariable
except NameError:
    print("well, it WASN'T defined after all!")
else:
    print("sure, it was defined.")

Jarlathus

@Aaron: There are many cases when you don't know whether variable is defined. You can refactor code to avoid this in many, but not all cases. Alex's solution is correct and it the best when refactoing is not possible for some reasons. There is not much information in the question, so I believe only person asked it can select the best way to handle his case. – Sargent

@Aaron, "should" is a 4-letter word -- e.g. no driver "should" ever exceed the speed limit, but that doesn't mean you don't take all proper and needed precautions against those who nevertheless do. Maintaining fragile, undertested legacy code with somewhat-broken design that you inherited from somebody else is a fact of life, and those who can only think of big-bang rewriting from scratch rather than cautiously and incrementally need to re-read Joel's 9-years-old essay joelonsoftware.com/articles/fog0000000069.html . – Jarlathus

@Radii People use it to maintain backwards compatibility. Search for NameError in Python 3.1 source code. There are many instances where "try: var_name except NameError: something_else" is used. Here are a couple of place where it is used: CSV (svn.python.org/projects/python/trunk/Lib/csv.py) library and in the ElementTree library (svn.python.org/projects/python/trunk/Lib/xml/etree/…). – Entire

This debate is much more interesting that the answer itself, which by the way is 100% correct and let you handle poor legacy code elegantly. – Cordiecordier

it depends on the context, in a scripting environment, for example, it's perfectly fine to check if a variable is defined or not, and ad. es. give it a default value. my 2c. – Wariness

@Radii - What about a variable in a view template, where it's up to the including code to set the variable? How would you ensure a defined variable in those situations? – Translative

Variables can also be dynamically defined and I don't want my script to stop working just because the variable gets correctly defined only if "x" happens. Upvote! – Rhythmist

Note: If it's a class attribute, you need to use try: self.MyMissingVar except AttributeError: print 'It's not there!' – Philpot

Much of the discussion in the other answers assumes that the problem is how to tell whether a symbol has an initial value, whereas the OP made it clear that the question is really whether the symbol has any value at all. None is a value, so a symbol whose value is None DOES have a value "at all". – Ridden

[continuing] That is why using a try or checking globals() is necessary. Example: You have a file that performs significant initialization computation whose results you want to assign to global symbols. You want to be able to execfile/exec (Python 2/3) the file from any of a number of modules, some perhaps used in more than one program. However, you only want the initialization performed once during a program's execution. Solution: put the execfile/exec in as many places as you want and wrap the file's contents in a conditional that tests whether one of the symbols it defines has a value. – Ridden

Exceptions should be reserved for exceptional cases, not for use as the bastard brother of 'if' :-( – Lizettelizotte

See also a general discussion on when to use exceptions here on SO. – Lizettelizotte

Meta discussion linking to this answer meta.https://mcmap.net/q/47492/-where-can-i-find-the-maven-installation-directory-in-eclipse-3-4/73226 – Aerodynamics

@einpoklum, Python uses exception StopIteration within just about every for statement -- that's how an iterator lets it known that it's all done. And of course, it is far from "an exceptional case" for iteration to be done -- indeed, one expects most iterations to terminate. Thus, obviously, your opinions on how exceptions "should" be used are not correctly applicable to Python (other languages have different pragmatics and the question cannot be properly treated as "language agnostic" as in the Q you point to). – Jarlathus

I agree with @AlexMartelli: The ability to have undefined variables is a feature in Python. As any features, it has some use cases where it is more or less appropriate. But it it certainly not never appropriate. Testing for it being defined or not is not less strange than testing for the existence of a file. And Python being a lot about duck typing, using an exception makes perfect sense. And by the way, exceptions are the bastard brothers of if:s. – Forrester

@habnabit Easy refutal: I'm using iniparse to load a config file but want to make sure certain variables were set, or the program can't continue. – Baillie

stumbled across this problem while using pyspark with Jupiter (which DOES define SparkContext as sc for you) and Mesos, which does not. This is why we need this sometimes. – Odaodab

python is used for interactive data analysis in say Jupyter notebooks. In this case, it's great to be able to add a check for whether a variable is defined or not before potentially reloading or recomputing a costly variable simply because you changed another portion of code in the cell and reevaluated it. – Pollock

@Shadur irrelevant; iniparse doesn't set variables. – Catacaustic

@Lizettelizotte in python try/except is not exceptional it is as friendly a part of program control as if or for. – Atchison

Interesting answer, as I have never known that you could use else in a try block. I still don't see the advantage of that over just placing the else-line as the second line in the try section. – Brigidbrigida

Answer 2

'a' in vars() or 'a' in globals()

if you want to be pedantic, you can check the builtins too
'a' in vars(__builtins__)

Heroin

This should be the answer I believe. see #843777 – Abbott

This is a very idiomatic answer. There is a strong use case here for maps; for example, if "prop2" in {"prop0": None, "prop1": None}: – Gynecium

This answer also works for detecting members other than variables, such as functions, classes, and modules. – Bleach

As of today — June 2018: I don't think vars(__builtin__) still works (either for Python 2 or 3). I believe vars(__builtins__) (mind the plural ;-)) should be used for both versions now (tested on: Python 2.7.10 and Python 3.6.1). – Instalment

Why not locals() too, why not dir()? – Unwarrantable

@Unwarrantable globals() is a superset of locals(). – Exultation

This should be the accepted answer. It looks much better when embedded into an if structure that checks for other conditions. – Huppert

I like this one better: a = globals().get('a','@') Thanks @Gnosticism – Stamey

Answer 3

I think it's better to avoid the situation. It's cleaner and clearer to write:

a = None
if condition:
    a = 42

Microeconomics

If you like to do it that way you should make the initial value unique so you can distinguish from something setting a to None ie. UNDEFINED=object();a=UNDEFINED then you can test with a is not UNDEFINED – Heroin

This is a common use of None. It's literally the semantic meaning of None-- it's nothing, nada, zip. It exists, but has "no value". Using a non-None value is useful if None can come up in an assignment and so on, but this is not so common that avoiding None should be standard practice. For example, in the question, a would only ever be 42 or undefined. Changing undefined to having a value of None results in no loss of information. – Noiseless

This is THE way, not sure why people think using a LONG and DIRTY try/except construct to get this done. ALSO this is very useful when using keyword args in class member functions where you don't want to hard-code the default values into the parameter list (you just define the default value as a class member). – Kamseen

@DevinJeanpierre That's not necessarily true. If you want to create some client library for a JSON API that treats a null value as different from an unspecified value (eg. None sets the actual JSON value to null, whereas an unspecified leaves it out of the JSON message entirely, which makes the API use a default or calculate from other attributes), you need to either differentiate None from Undefined, or None from JSON null. It's incredibly short-sighted to think that None can't or shouldn't ever be used as distinct from a value that is explicitly unspecified or undefined. – Lyophobic

@Lyophobic sure. "Using a non-None value is useful if None can come up in an assignment and so on, but this is not so common that avoiding None should be standard practice." If a non-None value can come up, then None is not acceptable, and this whole pattern is error-prone. – Noiseless

Providing a default value (which is what this answer is doing) is far different than testing the existence of a variable. There may be much overlap in the use cases, but if you want to see if something has been defined, you'll never know with this method. – Dipeptide

An even better solution is: if condition: TeXx = Object(); else: TeXx = None; something.setEnabled(isinstance(TeXx, xmlDOM));; because let's say I redefine the object as a str or file, not only do I check for None, but it's the right is not None – Termitarium

This isn't a good answer because there are situations where this doesn't work like trying get a value from a dictionary. – Tollbooth

Answer 4

try:
    a # does a exist in the current namespace
except NameError:
    a = 10 # nope

Entire

Exceptions should only be used in exceptional cases. See this discussion – Lizettelizotte

@Lizettelizotte This is true for most languages, but in Python exceptions are used more often, see e.g. stackoverflow.com/questions/3086806 – Crinite

Hmm, strange but true, apparently. I still think that's not a Good Idea (TM) but, well, can't argue with community customs. – Lizettelizotte

Better use the if 'a' in vars() or 'a' in globals() solution mentioned above, which does not require an exception – Laryngitis

try/except is what you do in python. what is a code smell in one language is a feature in another. – Atchison

Python definitely encourages you to use exceptions more frequently. In many situations instead of specifically checking if an action is possible you instead should just catch the exception. Do this cleanly by making sure you're only catching exactly the exception type you'd expect. The above answer is a great pythonic solution. (Also exceptions in Python are cheap with little overhead) – Deuterogamy

Answer 5

For this particular case it's better to do a = None instead of del a. This will decrement reference count to object a was (if any) assigned to and won't fail when a is not defined. Note, that del statement doesn't call destructor of an object directly, but unbind it from variable. Destructor of object is called when reference count became zero.

Sargent

Answer 6

One possible situation where this might be needed:

If you are using finally block to close connections but in the try block, the program exits with sys.exit() before the connection is defined. In this case, the finally block will be called and the connection closing statement will fail since no connection was created.

Vernitavernoleninsk

Then perhaps the connection close should not be done in the finally? – Comparable