See the Rationale section from PEP 424 that introduced __length_hint__ and offers insight on the motivation: Being able to pre-allocate lists based on the expected size, as estimated by __length_hint__ , can be a significant optimization. CPython has been observed to run some code faster than PyPy, purely because of this optimization being present. In … Read more
Immutable objects don’t have the same id, and as a matter of fact this is not true for any type of objects that you define separately. Generally speaking, every time you define an object in Python, you’ll create a new object with a new identity. However, for the sake of optimization (mostly) there are some … Read more
Python separates the right-hand side expression from the left-hand side assignment. First the right-hand side is evaluated, and the result is stored on the stack, and then the left-hand side names are assigned using opcodes that take values from the stack again. For tuple assignments with 2 or 3 items, Python just uses the stack … Read more
The difference is that reversed is an iterator (it’s also lazy-evaluating) and sorted is a function that works “eagerly”. All built-in iterators (at least in python-3.x) like map, zip, filter, reversed, … are implemented as classes. While the eager-operating built-ins are functions, e.g. min, max, any, all and sorted. >>> a = [1,2,3,4] >>> r … Read more
I know lists use C arrays under the hood which made me conclude that lookup in a list with just a few items would be better than in a dictionary (accessing a few elements in an array is faster than computing a hash). Accessing a few array elements is cheap, sure, but computing == is … Read more
Note: This answer was written for the Python 3.6 alpha releases. A new opcode added to 3.6.0b1 improved f-string performance significantly. The f”…” syntax is effectively converted to a str.join() operation on the literal string parts around the {…} expressions, and the results of the expressions themselves passed through the object.__format__() method (passing any :.. … Read more
tl;dr: As the reference manual states: A block is a piece of Python program text that is executed as a unit. The following are blocks: a module, a function body, and a class definition. Each command typed interactively is a block. This is why, in the case of a function, you have a single code … Read more
I’m going to mention one of the new features of Python 3.6 – f-strings. They can evaluate expressions, >>> eval(‘f”{().__class__.__base__}”‘, {‘__builtins__’: None}, {}) “<class ‘object’>” but the attribute access won’t be detected by Python’s tokenizer: 0,0-0,0: ENCODING ‘utf-8’ 1,0-1,1: ERRORTOKEN “‘” 1,1-1,27: STRING ‘f”{().__class__.__base__}”‘ 2,0-2,0: ENDMARKER ”
Both work differently. The list comprehension version takes advantage of the special bytecode LIST_APPEND which calls PyList_Append directly for us. Hence it avoids an attribute lookup to list.append and a function call at the Python level. >>> def func_lc(): [x**2 for x in y] … >>> dis.dis(func_lc) 2 0 LOAD_CONST 1 (<code object <listcomp> at … Read more
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 … Read more