Is it possible only to declare a variable without assigning any value in Python?

Question

Is it possible to declare a variable in Python, like so?:

var

so that it initialized to None? It seems like Python allows this, but as soon

Answer 1

Why not just do this:

var = None

Python is dynamic, so you don't need to declare things; they exist automatically in the first scope where they're assigned. So, all you need is a regular old assignment statement as above.

This is nice, because you'll never end up with an uninitialized variable. But be careful -- this doesn't mean that you won't end up with incorrectly initialized variables. If you init something to None, make sure that's what you really want, and assign something more meaningful if you can.

Answer 2

Well, if you want to check if a variable is defined or not then why not check if its in the locals() or globals() arrays? Your code rewritten:

for index in sequence:
   if 'value' not in globals() and conditionMet:
       value = index
       break

If it's a local variable you are looking for then replace globals() with locals().

Answer 3

I usually initialize the variable to something that denotes the type like

var = ""

or

var = 0

If it is going to be an object then don't initialize it until you instantiate it:

var = Var()

Answer 4

You look like you're trying to write C in Python. If you want to find something in a sequence, Python has builtin functions to do that, like

value = sequence.index(blarg)

Answer 5

If None is a valid data value then you need to the variable another way. You could use:

var = object()

This sentinel is suggested by Nick Coghlan.

Answer 6

First of all, my response to the question you've originally asked

Q: How do I discover if a variable is defined at a point in my code?

A: Read up in the source file until you see a line where that variable is defined.

But further, you've given a code example that there are various permutations of that are quite pythonic. You're after a way to scan a sequence for elements that match a condition, so here are some solutions:

def findFirstMatch(sequence):
    for value in sequence:
        if matchCondition(value):
            return value

    raise LookupError("Could not find match in sequence")

Clearly in this example you could replace the raise with a return None depending on what you wanted to achieve.

If you wanted everything that matched the condition you could do this:

def findAllMatches(sequence):
    matches = []
    for value in sequence:
        if matchCondition(value):
            matches.append(value)

    return matches

There is another way of doing this with yield that I won't bother showing you, because it's quite complicated in the way that it works.

Further, there is a one line way of achieving this:

all_matches = [value for value in sequence if matchCondition(value)]