cpython

It is standard convention to use if foo is None rather than if foo == None to test if a value is specifically None . If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True ? Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.? Example: say True is used as a singleton value that you want to differentiate from the value 'bar' , or any other true-like value: if foo is True: # vs foo == True ... elif foo == 'bar': ... Is there a case where using if foo is True

I have encountered this weird behavior and failed to explain it. These are the benchmarks: py -3 -m timeit "tuple(range(2000)) == tuple(range(2000))" 10000 loops, best of 3: 97.7 usec per loop py -3 -m timeit "a = tuple(range(2000)); b = tuple(range(2000)); a==b" 10000 loops, best of 3: 70.7 usec per loop How come comparison with variable assignment is faster than using a one liner with temporary variables by more than 27%? By the Python docs, garbage collection is disabled during timeit so it can't be that. Is it some sort of an optimization? The results may also be reproduced in Python 2.x

I was playing around with timeit and noticed that doing a simple list comprehension over a small string took longer than doing the same operation on a list of small single character strings. Any explanation? It's almost 1.35 times as much time. >>> from timeit import timeit >>> timeit("[x for x in 'abc']") 2.0691067844831528 >>> timeit("[x for x in ['a', 'b', 'c']]") 1.5286479570345861 What's happening on a lower level that's causing this? Veedrac TL;DR The actual speed difference is closer to 70% (or more) once a lot of the overhead is removed, for Python 2. Object creation is not at fault.

I want to access some .NET assemblies written in C# from Python code. A little research showed I have two choices: IronPython with .NET interface capability/support built-in Python with the Python .NET package What are the trade-offs between both solutions? Reed Copsey If you want to mainly base your code on the .NET framework, I'd highly recommend IronPython vs Python.NET. IronPython is pretty much native .NET - so it just works great when integrating with other .NET langauges. Python.NET is good if you want to just integrate one or two components from .NET into a standard python application.

I'm looking into these two related questions: here and here . I am seeing a behavior I do not expect in Python 3.6, which differs from behavior using plain reload in Python 2.7 (and 3.4). Namely, it seems that a module attribute that would be populated during module initialization or when re-exec-ing the module during a reload, is not restored after its local name is removed with del ... see below: For Python 3.6: In [1]: import importlib In [2]: import math In [3]: del math.cos In [4]: math.cos --------------------------------------------------------------------------- AttributeError

简评：编写安全代码很困难，当你学习一个编程语言、模块或框架时，你会学习其使用方法。 在考虑安全性时，你需要考虑如何避免被滥用，Python 也不例外，即使在标准库中，也存在用于编写应用的不良实践。然而，许多 Python 开发人员却根本不知道它们。 接上篇 6. 解析 XML（Parsing XML） 如果你的应用程序要加载、解析 XML 文件，则你可能正在使用 XML 标准库模块。通过 XML 的攻击大多是 DoS 风格（旨在使系统崩溃而不是泄露数据），这些攻击十分常见，特别是在解析外部（即不可信任的）XML 文件时。 其中有个「billion laughs」，因为他的 payload 通常包含很多（十亿）「lols」。基本上，这个原理是可以在 XML 中使用参照实体，所以当解析器将这个 XML 文件加载到内存中时，它会消耗数 G 大小的内存（RAM）。 试试看，如果你不相信我的话 :-) <?xml version="1.0"?> <!DOCTYPE lolz [ <!ENTITY lol "lol"> <!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;"> <!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;