python-internals

Why in Python it is evaluated this way: >>> False is False is False True but when tried with parenthesis is behaving as expected: >>> (False is False) is False False Chaining operators like a is b is c is equivalent to a is b and b is c . So the first example is False is False and False is False , which evaluates to True and True which evaluates to True Having parenthesis leads to the result of one evaluation being compared with the next variable (as you say you expect), so (a is b) is c compares the result of a is b with c . Quoting Python official documentation , Formally, if a , b , c , ...

I wonder why a class __dict__ is a mappingproxy , but an instance __dict__ is just a plain dict >>> class A: ... pass >>> a = A() >>> type(a.__dict__) <class 'dict'> >>> type(A.__dict__) <class 'mappingproxy'> This helps the interpreter assure that the keys for class-level attributes and methods can only be strings. Elsewhere, Python is a "consenting adults language", meaning that dicts for objects are exposed and mutable by the user. However, in the case of class-level attributes and methods for classes, if we can guarantee that the keys are strings, we can simplify and speed-up the common

I'm seeing some inconsistencies when using sys.getsizeof on what should be identical lists. (Python 2.7.5) >>> lst = [0,1,2,3,4,5,6,7,8,9] >>> sys.getsizeof(lst) 76 >>> lst2 = list(lst) >>> sys.getsizeof(lst2) 104 >>> lst3 = list(lst2) >>> sys.getsizeof(lst3) 104 >>> sys.getsizeof(lst[:]) 76 >>> sys.getsizeof(lst2[:]) 76 Does anybody have a simple explanation? With a list literal, the VM creates the list with a set length. When passing a sequence to the list() constructor the elements are added one by one (via list.extend() ) and as such the list is resized when appropriate. Since the resize

This is more of a conceptual question. I recently saw a piece of code in Python (it worked in 2.7, and it might also have been run in 2.5 as well) in which a for loop used the same name for both the list that was being iterated over and the item in the list, which strikes me as both bad practice and something that should not work at all. For example: x = [1,2,3,4,5] for x in x: print x print x Yields: 1 2 3 4 5 5 Now, it makes sense to me that the last value printed would be the last value assigned to x from the loop, but I fail to understand why you'd be able to use the same variable name for

I can create a multi-line string using this syntax: string = str("Some chars " "Some more chars") This will produce the following string: "Some chars Some more chars" Is Python joining these two separate strings or is the editor/compiler treating them as a single string? P.s: I just want to understand the internals. I know there are other ways to declare or create multi-line strings. Read the reference manual , it's in there . Specifically: Multiple adjacent string or bytes literals (delimited by whitespace), possibly using different quoting conventions, are allowed, and their meaning is the