How do I check if a variable exists?

To check the existence of a local variable:

if 'myVar' in locals():
  # myVar exists.

To check the existence of a global variable:

if 'myVar' in globals():
  # myVar exists.

To check if an object has an attribute:

if hasattr(obj, 'attr_name'):
  # obj.attr_name exists.

The use of variables that have yet to been defined or set (implicitly or explicitly) is often a bad thing in any language, since it tends to indicate that the logic of the program hasn't been thought through properly, and is likely to result in unpredictable behaviour.

If you need to do it in Python, the following trick, which is similar to yours, will ensure that a variable has some value before use:

try:
    myVar
except NameError:
    myVar = None      # or some other default value.

# Now you're free to use myVar without Python complaining.

However, I'm still not convinced that's a good idea - in my opinion, you should try to refactor your code so that this situation does not occur.

By way of an example, the following code was given below in a comment, to allow line drawing from a previous point to the current point:

if last:
    draw(last, current);
last = current

In the case where last has not been bound to a value, that won't help in Python at all since even the checking of last will raise an exception. A better idea would be to ensure last does have a value, one that can be used to decide whether or not it is valid. That would be something like:

last = None

# some time passes ...

if last is not None:
    draw(last, current);
last = current

That ensures the variable exists and that you only use it if it's valid for what you need it for. This is what I assume the if last was meant to do in the comment code (but didn't), and you can still add the code to force this if you have no control over the initial setting of the variable, using the exception method above:

# Variable 'last' may or may not be bound to a value at this point.

try:
    last
except NameError:
    last = None

# It will always now be bound to a value at this point.

if last is not None:
    draw(last, current);
last = current

Using try/except is the best way to test for a variable's existence. But there's almost certainly a better way of doing whatever it is you're doing than setting/testing global variables.

For example, if you want to initialize a module-level variable the first time you call some function, you're better off with code something like this:

my_variable = None

def InitMyVariable():
  global my_variable
  if my_variable is None:
    my_variable = ...

catch is called except in Python. other than that it's fine for such simple cases. There's the AttributeError that can be used to check if an object has an attribute.

I created a custom function.

def exists(var):
     return var in globals()

Then the call the function like follows replacing variable_name with the variable you want to check:

exists("variable_name")

Will return True or False

Like so:

def no(var):
    "give var as a string (quote it like 'var')"
    assert(var not in vars())
    assert(var not in globals())
    assert(var not in vars(__builtins__))
    import keyword
    assert(var not in keyword.kwlist)

Then later:

no('foo')
foo = ....

If your new variable foo is not safe to use, you'll get an AssertionError exception which will point to the line that failed, and then you will know better. Here is the obvious contrived self-reference:

no('no')

---------------------------------------------------------------------------
AssertionError                            Traceback (most recent call last)
<ipython-input-88-d14ecc6b025a> in <module>
----> 1 no('no')

<ipython-input-86-888a9df72be0> in no(var)
      2     "give var as a string (quote it)"
      3     assert( var not in vars())
----> 4     assert( var not in globals())
      5     assert( var not in vars(__builtins__))
      6     import keyword

AssertionError: 

It may not be performant, but you generalise the solution to a function that checks both local variables and global variables.

import inspect
def exists_var(var_name):
    frame = inspect.currentframe()
    try:
        return var_name in frame.f_back.f_locals or var_name in globals()
    finally:
        del frame

Then you can use it like this:

exists_var('myVar')

Short variant:

my_var = some_value if 'my_var' not in globals() else my_var:

This was my scenario:

for i in generate_numbers():
    do_something(i)
# Use the last i.

I can’t easily determine the length of the iterable, and that means that i may or may not exist depending on whether the iterable produces an empty sequence.

If I want to use the last i of the iterable (an i that doesn’t exist for an empty sequence) I can do one of two things:

i = None  # Declare the variable.
for i in generate_numbers():
    do_something(i)
use_last(i)

or

for i in generate_numbers():
    do_something(i)
try:
    use_last(i)
except UnboundLocalError:
    pass  # i didn’t exist because sequence was empty.

The first solution may be problematic because I can’t tell (depending on the sequence values) whether i was the last element. The second solution is more accurate in that respect.

Also a possibility for objects, use __dict__.

class A(object):
    def __init__(self):
        self.m = 1

a = A()
assert "m" in a.__dict__
assert "k" not in a.__dict__