[Tutor] How python keeps track of data types
Hello I am a computer science student,I want to know how python keeps track of data types of variables and is there a way to get data types of variables without actually running the script(with type() function). I think python AST can help, but am unable to figure out.Please help me. I've explored compiler library but could not find anything helpful. Thanks Abhishek Kumar ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] How python keeps track of data types
On 28/03/16 20:31, Abhishek Kumar wrote: > I am a computer science student,I want to know how python keeps track of > data types of variables and is there a way to get data types of variables > without actually running the script(with type() function). First, recall that Python variables are just names so they don't really have a type as such, it's the objects that they reference that have types. Second, I'm not sure what you mean by "get data types of variables without actually running the script"? The variables/objects don't exist until you run the script through the interpreter. Python is not statically typed so you cannot analyse the source code in any meaningful way to get the information. Can you explain what you want to do with a short example. For example if I have a script hello.py like ## import sys message = sys.version greeting = "Hello there, " print greeting + message message = 42 print "the answer is ", message ## Now how would you want to examine that to determine the "types" of sys, message and greeting?? Especially since message takes on two types(objects) during the execution of the code. > AST can help, but am unable to figure out.Please help me. I don't see how AST would help in this case. It looks at the syntax and in Python there are no type declarations in the syntax. (The new type hints feature might help but very few people actually use that and it doesn't apply in all scenarios) -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] How python keeps track of data types
Abhishek Kumar writes: > I am a computer science student,I want to know how python keeps track > of data types of variables Python doesn't track the type of a variable, because a variable never has any information about type. What Python calls “variables” are names, referring to objects. Names have no type. Every object has a type, so Python doesn't “track” the type in the sense you mean. -- \ “Any sufficiently advanced bug is indistinguishable from a | `\ feature.” —Rich Kulawiec | _o__) | Ben Finney ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] How python keeps track of data types
On Tue, Mar 29, 2016 at 01:01:57AM +0530, Abhishek Kumar wrote: > Hello > I am a computer science student,I want to know how python keeps track of > data types of variables and is there a way to get data types of variables > without actually running the script(with type() function). I think python > AST can help, but am unable to figure out.Please help me. > I've explored compiler library but could not find anything helpful. Variables in Python do not have types like in C, Pascal or Haskell. Variables can take values of any type: x = None x = 42 # x is now an int x = "Hello" # now x is a str x = [1, 2, 3] # a list of ints x = [1, None, "Hello", 2.5] # list of mixed types x = lambda arg: arg + 1 # and now a function So Python doesn't track the type of variables at all, since variables do not have types. Only values have types: None # the special "None" singleton has type NoneType 42 # an int "Hello" # a string 2.5 # a float and so forth. How does Python track the type of values? In Python, all values are objects. Even strings, floats, ints and None. Objects are typically implemented as a C struct, with a pointer back to the class. So the interpreter can tell the type of the object by looking at the pointer to the class. But that is not part of the Python language, that is part of the implementation. Different Python interpreters, and different versions, will work differently. For example, Jython uses the Java Virtual Machine, so all objects in Jython are Java objects. Jython lets the JVM track the type of objects, using whatever method the JVM uses. Likewise, IronPython uses the .Net Common Language Runtime, so all objects in IronPython are CLR objects. IronPython lets .Net (or Mono) track the type of objects. The "reference implementation" of Python is the normal version of Python most people use, sometimes called CPython. Remember that other versions of Python may work differently inside the interpeter, but the language features should be the same. Python's data model is explained here: https://docs.python.org/3/reference/datamodel.html The CPython implementation is explained in part here: https://docs.python.org/3/c-api/intro.html but only the parts which are public and part of the C API. To really understand the inner workings, you must read the source code: https://hg.python.org/cpython/ The way to determine the type of an object in Python is to call the type() function. -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
