关于`kur.engine.Engine._evaluate`如何工作的困惑

Question

问题

在使用kur.engine.Engine._evaluate方面似乎存在冲突。

当我在kur dump mnist.yml目录下运行kur/example/并查看运行详细信息时，我得到了：

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然而，当我运行eng = kur.engine.Engine()和eng._evaluate('mnist-defaults.yml')时，我得到了以下错误：

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源代码说什么？

我读取这个函数doc: 1.将字符串作为arg命名的表达式2.返回python对象或类

我阅读这段代码: 1.给函数一个字符串arg 2。它返回一个名为“未实现的错误”的错误

def _evaluate(self, expression):
    """ Evaluates a string expression in the current scope.

        # Arguments

        expression: str. The string to evaluate.

        # Return value

        The evaluated expression (some Python object/class)
    """
    # pdb.set_trace()
    raise NotImplementedError

但是，实际上，当在控制台kur dump mnist.yml中运行时，它的工作方式就像文档所说的那样。那么，我的理解哪里出错了？谢谢!

Answer 1

Engine是抽象基类。这意味着它是，而不是打算直接实例化的。相反，它的派生类使用基类作为扩展其功能的“模板”(可以这么说)。这是面向对象程序设计中的一个典型模式，特别是在Java和C++等语言中。Python是一种非常灵活的语言，有许多方法可以实现类似的设计模式(例如，abc模块、鸭子类型或经典继承加上良好的文档和NotImplementedError)。

在特定情况下，当实际运行Kur时，引擎的实例化(运行时)类型不是Engine，而是JinjaEngine。因此，当进行_evaluate()调用时，处理它的实际上是JinjaEngine，而不是基类(正如您正确指出的那样，它会引发异常)。尝试实例化一个JinjaEngine，您应该得到预期的结果。

Answer 2

基于上面的@ajsyp评论：

您肯定是在实例化引擎，这是一个抽象基类，不应该这样做。当Kur实际运行时，或者如果您想得到预期的行为，那么实例化一个JinjaEngine。抽象基类是一种表示“这是这种类型的对象的行为方式”的方式，而不是实际创建一个对象；相反，派生类负责实现基类的行为。这是一个标准的OOP模式

如果我正确地理解了在运行kur -vv build mnist.yml和使用记录器在kur.Kurfile.parse_source下显示工作中的函数时，我注意到了以下几点：

[INFO 2017-04-11 14:05:05,946 kur.kurfile parse:97] (self): after initialize Kurfile object, we parse it: 1. evaluate all section dicts in spec.data with scopes; 2. as a result, spec.data added section aliases (like training, testing);3. other uses here to be answered ....; 4. assign spec.data['templates'] to spec.templates; 3. convert spec.data['model'] into model as containers, and store the list of containers inside spec.contaienrs ; 5. return Nothing
[INFO 2017-04-11 14:05:05,946 kur.kurfile _parse_section:1006]
[DEBUG 2017-04-11 14:05:05,946 kur.kurfile _parse_section:1011] _parse_section(): Parsing Kurfile section: settings
[INFO 2017-04-11 14:05:05,947 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,947 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,947 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,947 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,947 kur.kurfile _parse_section:1006]
[DEBUG 2017-04-11 14:05:05,947 kur.kurfile _parse_section:1011] _parse_section(): Parsing Kurfile section: train
[INFO 2017-04-11 14:05:05,947 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,948 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,949 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,949 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,950 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,950 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,951 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,951 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,951 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,951 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,952 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,952 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.
[INFO 2017-04-11 14:05:05,952 kur.engine.jinja_engine _evaluate:123] (self, expression): convert a yml expression (a string) to a python object
[INFO 2017-04-11 14:05:05,953 kur.engine.engine evaluate:209] (self, expression, recursive=False): Evaluates an string expression in the current scope, as itself: (how does scope help evaluate expression? Not sure how is scope used); if expression is not a string, then do some recursion.

1. 当代码在kur.Kurfile.parse上运行时，它在settings、train、validate等不同的部分进行解析。
2. 在执行kur.Kurfile._parse_section时，大多数部分都需要运行大量的kur.engine.engine.Engine.evaluate，如果不是，只需要运行几个kur.engine.jinjiaEngine.JinjaEngine._evaluate

我是否应以下列方式理解该评论和意见：

1. kur.engine.engine.Engine.evaluate，kur.engine.engine.Engine._evaluate是基类方法，实际上我不需要理解它们的内部，对吗？
2. kur.engine.jinja_engine.JinjaEngine._evaluate是从上述基本方法导出的方法，我需要理解它的实际意义，对吗？
3. 我对kur.engine.jinja_engine.JinjaEngine._evaluate的理解是:将yaml字符串表达式转换为python对象。这是正确的吗？

如果没有，你能举个例子解释一下吗？(不必是工作代码，只是一个我们可以在脑海中想象的例子也可以)

谢谢

@ajsyp更多的答案和解释非常有用

1.)根据定义，Engine类中定义的所有函数都是基类方法。这并不意味着，而不是，它们并不重要。它们是极其重要的！具体来说，Engine.evaluate()确实有效，它试图弄清楚如何处理数据、它是什么类型、是否需要递归等等。Engine._evaluate定义了派生类必须实现的API (不过，从纯代码的角度来看，Engine._evaluate()并不做任何事情，因为它的工作是定义，并且所有的实现都委托给派生类)。2.)是的，JinjaEngine._evaluate()是一个派生类方法。在这种情况下，它实现了由父类的API定义的逻辑。3.)有点。JinjaEngine._evaluate()负责使用Kurfile的当前值来计算Jinja2表达式(而不是YAML表达式；YAML解析器已经这样做了；实际上，您本来可以使用JSON，而整个答案仍然是正确的)。Jinja2表达式的计算结果是一个字符串表达式。因此，如果有必要，函数还需要解释该字符串，以构造底层Python类型(例如，整数4与字符串"4")。