Sympy TypeError：转储到 YAML 时无法确定关系的真值

Question

尝试将 sympy 表达式转储到 YAML 时出现此错误：

TypeError: cannot determine truth value of Relational

我要转储的是包含 2 个 sympy.Symbol 对象的集合。我不确定为什么 sympy 试图将其评估为 StrictLessThan 对象。 sympy 试图确定真值，也许是为了评估 mapping.sort() 函数？

这是回溯：

Traceback (most recent call last):
  File "E:/Matt/Documents/ProgrammingProjects/infinitree/abg.py", line 267, in <module>
    print yaml.dump(mtree)
  File "C:\Python27\lib\site-packages\yaml\__init__.py", line 202, in dump
    return dump_all([data], stream, Dumper=Dumper, **kwds)
  File "C:\Python27\lib\site-packages\yaml\__init__.py", line 190, in dump_all
    dumper.represent(data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 28, in represent
    node = self.represent_data(data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 61, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 434, in represent_object
    u'tag:yaml.org,2002:python/object:'+function_name, state)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 61, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 434, in represent_object
    u'tag:yaml.org,2002:python/object:'+function_name, state)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 223, in represent_dict
    return self.represent_mapping(u'tag:yaml.org,2002:map', data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 61, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 434, in represent_object
    u'tag:yaml.org,2002:python/object:'+function_name, state)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 61, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 434, in represent_object
    u'tag:yaml.org,2002:python/object:'+function_name, state)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 223, in represent_dict
    return self.represent_mapping(u'tag:yaml.org,2002:map', data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 223, in represent_dict
    return self.represent_mapping(u'tag:yaml.org,2002:map', data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 61, in represent_data
    node = self.yaml_multi_representers[data_type](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 434, in represent_object
    u'tag:yaml.org,2002:python/object:'+function_name, state)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 123, in represent_mapping
    node_value = self.represent_data(item_value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 57, in represent_data
    node = self.yaml_representers[data_types[0]](self, data)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 229, in represent_set
    return self.represent_mapping(u'tag:yaml.org,2002:set', value)
  File "C:\Python27\lib\site-packages\yaml\representer.py", line 120, in represent_mapping
    mapping.sort()
  File "C:\Python27\lib\site-packages\sympy\core\relational.py", line 195, in __nonzero__
    raise TypeError("cannot determine truth value of Relational")
TypeError: cannot determine truth value of Relational

Answer 1

表示器尝试对集合进行排序，该集合的转储方式与 dict 类似，方法是按键值对对其项目进行排序，然后转储它们。我不知道sympy 的详细信息，但是对于排序，需要知道键和值的表示（对于一个集合，只使用键），这通常是使用字符串表示来完成的，我猜, 引起评价。所以mapping.sort() 评估它用作键的sympy 对象，而不是相反（正如您在问题中指出的那样）

然而，问题不仅仅是排序，sympy 对象不能在没有额外的表示例程的情况下被转储：

import sys
import ruamel.yaml
from sympy.abc import x, y
from  sympy.core.relational import Relational

data = Relational(y, x+x**2, '==')

ruamel.yaml.round_trip_dump(data, sys.stdout)

还有哪些错误：

ruamel.yaml.representer.RepresenterError: cannot represent an object: Eq(y, x**2 + x)

所以我会从简单的开始，为您使用并希望转储的对象类型创建表示器，然后这些对象的集合也应该是可转储的。

---

_{我使用了我的 ruamel.yaml 库，而不是仅支持 YAML 1.1 规范的过时 PyYAML。在这个问题上，两者的行为方式应该相同。}