Can you share what code you have so far as well as the website you are scraping? It compares for identity of objects instead for their value equivalence. So when x is found in List it already points to that item? Can't you at least set it to None at the top of the program and test for that? There are different ways to verify a file or directory exists, using functions as listed below. Then the user-program's bindings are in a well-controlled place, and testing for boundedness is an ordinary. The only valid and portable solution I had was to execute the command and see error code. Attempting to access the variable and catching the possible exception is so far the only general technique I've come across for testing a variable's definedness. In fact, at the interpreter all three functions give a reference to the same dictionary.
There's a on adding Mac and Unix support for os. I don't like this, because it complicates the code and opens up questions such as, should this confirm to the Singleton programming pattern? You should prefer to return None in your tryGetObject even though you could return anything that you can detect. Testing for an undefined global variable. I don't like that answer because it pollutes the global namespace. The name is never bound to the object, always to the reference to the object. To check this, we use Built-in library functions. Edge Out The Competition for your dream job with proven skills and certifications.
What if they happen to use a variable of the same name as a variable you're using in your interpreter? For Python in particular, no: such apps are fine but that programming style is unnecessary and indeed a bad style in Python. Is it really what you need? It's more than this solution. By simple I mean something like the which command which would be just perfect. It doesn't work quite so well on numbers your example works better , but is fine for lists, which is what I really needed anyway: something to detect empty lists. And is that a global? What if they happen to use a variable of the same name as a variable you're using in your interpreter? They're no different than SyntaxErrors.
Or store globals like this in some dictionary, and use dict. When you need to copy the object, you have do do it somehow more explicitly. But if you are nevertheless in this situation, you can make sure that a given variable is in fact defined as None, if nothing else by attempting to access it inside a try clause and assigning it the None object if the access raises a NameError exception. How do I check to see if a variable or object has already been defined? I wear a lot of hats - Developer, Database Administrator, Help Desk, etc. However, variables that are in neither locals nor globals may exist thanks to the nested scopes feature that is optional in Python 2. Can't you at least set it to None at the top of the program and test for that? It is considered unusual in Python not to know whether a variable has already been defined.
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. But this means that, both the variables have in them. If you reference a variable that hasn't yet been assigned a value not the same thing as asking whether a vrbl has an undefined value in Perl terms, but kinda close if you squint , Python raises UnboundLocalError if it's a local vrbl else NameError. For example, if variable1 and variable2 are the results of re. Usually they fail to work on Windows because you can execute without the file extension and that you can register new extension. But if you are nevertheless in this situation, you can make sure that a given variable is in fact defined as None, if nothing else by attempting to access it inside a 'try' block and assigning it the None object should it raise a NameError exception.
Jacobs wrote: More generally, many values are considered false: None, 0, and an empty map or sequence. Note: Using a variable which hasn't been assigned a value, is an error in Python. Note that a variable being set to None is different from it not existing. 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. Either way, getenv is just a wrapper around environ. Für etwaige weitere Informationen stehen Ihnen unsere sowie unsere Nutzungsbedingungen zur Verfügung.
Use the exec statement, and pass it an explicit dict for evaluation context. NameError and UnboundLocalError exceptions should be treated as bugs. Next point: If you call this method often, the subprocess solution is much slower, since a lot of processes need to spawned. I think the in-python solution without subprocess is much better. I also find it useful to add this to the top of the function: import sys; if sys. In adition to what Richard wrote. It's also the fastest, as the 'is' keyword only checks for object identity.
Warning: When assigning a variable you always copy only the reference. I had problems when implementing the above when the variable I was testing for was to be a global variable, i. Unsere Partner führen diese Informationen möglicherweise mit weiteren Daten zusammen, die Sie ihnen bereitgestellt haben oder die sie im Rahmen Ihrer Nutzung der Dienste gesammelt haben. The None is actually singleton object in Python. In fact, at the interpreter all three functions give a reference to the same dictionary. Initializing variables is better practice in Python or any other language. In these applications you define little languages for users which are not really programmers, but for whom you want to provide a way to hack in some simple commands for a dedicated task.
Fortunately, Python has allowed functions to have attributes for a while, which gives us this simple solution: def InitMyVariable : if InitMyVariable. But will a variable always be in globals or locals? For example, if I'm looking for all the links in the website , I'll just use the appropriate command and beautifullsoup retrieves them. It used object-oriented approach to check if file exist or not. You can use any other value suitable for your purposes to initialize undefined variables; for a powerful and interesting example, see. 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.
None of previous examples do work on all platforms. In these applications you define little languages for users which are not really programmers, but for whom you want to provide a way to hack in some simple commands for a dedicated task. It helps if the executable has something like a --version flag that is a quick no-op. Then the user-program's bindings are in a well-controlled place, and that's extremely bad style in Python, but you probably knew that. 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.