Fortunately, Python has allowed functions to have attributes for a while, which gives us this simple solution: def InitMyVariable : if InitMyVariable. Surely you have control over where your variables are initialized? For example, how can I 1. Maybe, depending on the compiler. I am very new to Python, and have been unable to figure out how to check if a variable exists or not. These points are minutiae, to be sure, but sometimes clarity can be improved this way. If there are more variables you want to check, if all var in locals for var in variable1, variable2, variable3 : You connect the two variables by a boolean operator. For example, a Python program could open a socket on the server and it accepts Python scripts sent over the network from a client machine.
The cannonical way to tell if a variable exists in Python is to try to access it and catch any resulting NameError. The first way is to call the built-in function hasattr object, name , which returns True if the string name is the name of one of the object's attributes, False if not. I don't think it can be done, in general, without invoking a load of black magic. I am setting dist to 1e9, because that is a larger value than any of the places in the galaxy will be far away. There are different ways to verify a file or directory exists, using functions as listed below. None of previous examples do work on all platforms.
I know that I'm being a bit of a necromancer here, but I stumbled across this question and the accepted solution didn't work for me for all cases Thought it might be useful to submit anyway. Can anyone tell me how to wrap the following in a function as to avoid typing this try, except sequence for every variable whose existence I want to check? So, most of the time you want to use hasattr to check if an attribute exists in an object. I thought this would have been done automatically. For example on Windows if python is well installed it's enough to execute 'file. Great, that's what I wanted. Unfortunately the function only checks whether the specified path is a file, but does not guarantee that the user has access to it.
Python doesn't have a specific function to test whether a variable is defined, since all variables are expected to have been defined before use - even if initially just assigned the None object. How is that check done? Anyone know how to deal with this? I think the in-python solution without subprocess is much better. If you need some more advanced functionality, then this is where you should go. Note, however, that if you choose this alternative, you have to code things in this order: the anomalous, error case first, then the normal, no-error case. Also, you don't want to use property as a variable name. I will assume that the test is going to be used in a function, similar to user97370's answer.
The second way is to try to access an attribute in an object and perform some other function if an AttributeError was raised. The following code finds the closest place to a position and rejects places that are too far away. What you actually want is to and the outcome of the two comparisons. It also only tells you that the file existed at the point in time you called the function. Throughout these examples we'll assume our current working directory has these files and directories in it: drwxr-xr-x 3 scott staff 102 Jan 12 10:01 dir -rw-r--r-- 1 scott staff 5 Jan 12 09:56 file. So when x is found in List it already points to that item? You can use any other value suitable for your purposes to initialize undefined variables; for a powerful and interesting example, see. The use of variables that haven't been defined or set implicitly or explicitly is almost always a bad thing in any language since it indicates that the logic of the program hasn't been thought through properly, and is likely to result in unpredictable behaviour.
The following code finds the closest place to a position and rejects places that are too far away. How is that check done? When I add a new element self. If you know for sure which namespace the variable is in i. However, type checking is not a common idiom in python since we tend to use 'duck typing': if your object can do what you want with it, don't bother to check if it is exactly of the class you expect. Which way is better, then? The following is an example of how it can happen. But this means that, both the variables have in them.
To check this, we use Built-in library functions. For example, an attribute might be attached to an object due to its parent class instead of the object itself. Extending my existance checking question, how does one check what type a variable has? For example, would setting dist to infinity work, and how is that done? Last Updated: Tuesday 1 st July 2014 With Python there are several methods which can be used to check if a file exists, in a certain directory. This is a handy function to check if a file exists, because it's a simple one liner. Thank you for your replies. Well, both doctrines have loyal supporters and both styles seem to be well-versed to deal with any real-world programming challenge.
Can't you at least set it to None at the top of the program and test for that? Although it should be said that this goes only for variables. I wanted to iterate over some predefined viewers and run the first that exists. Can't you at least set it to None at the top of the program and test for that? In the following code I have made a kludge that works, but I think that it would be clearer to check if closest exists and not have to initialize it in the first place. I am very new to Python, and have been unable to figure out how to check if a variable exists or not. Für etwaige weitere Informationen stehen Ihnen unsere sowie unsere Nutzungsbedingungen zur Verfügung. Extending my existance checking question, how does one check what type a variable has? Another way is then: try: self.