Advanced Throughout the article I've been mentioning how all of the above methods utilize the method, so I figured it would be useful to take a look at it. Note: Using a variable which hasn't been assigned a value, is an error in Python. Although, one valid use-case might be when you're doing multiple tests on the same file and want to avoid the overhead of the for each test. It's more that if you're the developer you ought to know whether variables are defined or not. Published: Wednesday 27 th February 2013 Since everything in Python is an object and objects have attributes fields and methods , it's natural to write programs that can inspect what kind of attributes an object has. Since every operating system is different, the data provided by os. This is a lower-level method that will provide you with detailed information about files, directories, sockets, buffers, and more.
Lyle Johnson Well, there are some issues of which scope you're talking about, but if you want to see if it's defined in the global scope, you could do this: if not globals. Solution In Python, all variables are expected to be defined before use. 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. True if it exists False if it doesn't You can use in for tuple, list, and set as well. There's a on adding Mac and Unix support for os. Auctions - buy the things you want at great prices Sincerely yours, Roman Suzi Remco Gerlich I don't think it can be done, in general, without invoking a load of black magic.
Checking if a Directory Exists Like the isfile method, os. It will be interesting to hear others. So if you have quite a few tests to do then this will help you do it more efficiently. Numbers versus Digits Be sure that when you use the str. For example on Windows if python is well installed it's enough to execute 'file.
As it happens, most of my code does preset names to None, but reading this thread made me wonder. In Python, is there a portable and simple way to test if an executable program exists? You may just want to use. On Mac you have to run open command. So when x is found in List it already points to that item? These may or may not be numbers. I had a similar reaction to Fredrik's statement. However, I still don't think that's a good idea - in my opinion, you should refactor your code so that this situation does not occur. To check if an existing file is executable, use , with the os.
And is that a global? It used object-oriented approach to check if file exist or not. Or for some other reason? As it happens, most of my code does preset names to None, but reading this thread made me wonder. I think you have to be careful with your terminology, whether something exists and something evaluates to False are two different things. Only in the core of a frequently used inner loop should you even care about the cost of exceptions. Anyway, I hope that clarifies my intentions. Are you suggesting setting to None is better as it makes explicit that there is nothing there, but this may change in the future? Surely you have control over where your variables are initialized? Note that None is really nothing magical, just a built-in object used by convention and returned by functions that exit without returning anything specific. These points are minutiae, to be sure, but sometimes clarity can be improved this way.
Checking if a file or directory exists using Python is definitely one of those cases. I don't like this, because it complicates the code and opens up questions such as, should this confirm to the Singleton programming pattern? Would the following be a satisfactory implementation? Checking if a Python String is a Digit in Python str. As long as I use x as a variable, not as a value, right? If you need some more advanced functionality, then this is where you should go. Great, that's what I wanted. Note that a variable being set to None is different from it not existing.
For example, an attribute might be attached to an object due to its parent class instead of the object itself. I will assume that the test is going to be used in a function, similar to user97370's answer. In this tutorial, we will learn how to determine whether a file or directory exists using Python. I had problems when implementing the above when the variable I was testing for was to be a global variable, i. But for 90% of the time you're dealing with directories and files, the os or os. 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. However, variables that are in neither locals nor globals may exist thanks to the nested scopes feature that is optional in Python 2.
The second way is to try to access an attribute in an object and perform some other function if an AttributeError was raised. 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. I think the in-python solution without subprocess is much better. To check this, we use Built-in library functions. Not only is it often slow if performance is any kind of concern , but if you're using a variable as part of your exec string, security becomes a concern.
Next point: If you call this method often, the subprocess solution is much slower, since a lot of processes need to spawned. Python has a very lightweight exception mechanism. Anyone know how to check if an element with a certain attribute exists without going through all elements? If you know for sure which namespace the variable is in i. Beware: C++ and Java have an immense overhead for exceptions. Für etwaige weitere Informationen stehen Ihnen unsere sowie unsere Nutzungsbedingungen zur Verfügung. This works provided the executable has an option like --help or --version that outputs something and returns an exit status of zero.