无法在 google-app-engine 中使用 collections.defaultdict()
Can't use collections.defaultdict() in google-app-engine
尝试使用 collections.defaultdict() 在 google-app-engine 中创建直方图：
class myDS(ndb.Model):
values = ndb.PickleProperty()
hist = ndb.PickleProperty()
class Handler:
my_ds = myDS()
my_ds.values = {}
my_ds.hist = defaultdict(lambda : 0)
并得到错误（来自日志）
File "/base/alloc/tmpfs/dynamic_runtimes/python27/277b61042b697c7a_unzipped/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 1331, in call
newvalue = method(self, value)
File "/base/alloc/tmpfs/dynamic_runtimes/python27/277b61042b697c7a_unzipped/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 1862, in _to_base_type
return pickle.dumps(value, pickle.HIGHEST_PROTOCOL)
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed
有什么办法解决这个问题吗？
PickleProperty 字段需要一个可使用 Python 的 pickle 协议序列化的值（有关详细信息，请参阅 docs）：
PickleProperty: Value is a Python object (such as a list or a dict or a
string) that is serializable using Python's pickle protocol; Cloud
Datastore stores the pickle serialization as a blob. Unindexed by
default. Optional keyword argument: compressed.
另见来自 Martijn Pieters 的 answer：
Pickle cannot handle lambdas; pickle only ever handles data, not code,
and lambdas contain code. Functions can be pickled, but just like
class definitions only if the function can be imported. A function
defined at the module level can be imported. Pickle just stores a
string in that case, the full 'path' of the function to be imported
and referenced when unpickling again.
有多个选项可以使用默认值，具体取决于您的用例。
尝试使用 collections.defaultdict() 在 google-app-engine 中创建直方图：
class myDS(ndb.Model):
values = ndb.PickleProperty()
hist = ndb.PickleProperty()
class Handler:
my_ds = myDS()
my_ds.values = {}
my_ds.hist = defaultdict(lambda : 0)
并得到错误（来自日志）
File "/base/alloc/tmpfs/dynamic_runtimes/python27/277b61042b697c7a_unzipped/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 1331, in call
newvalue = method(self, value)
File "/base/alloc/tmpfs/dynamic_runtimes/python27/277b61042b697c7a_unzipped/python27_lib/versions/1/google/appengine/ext/ndb/model.py", line 1862, in _to_base_type
return pickle.dumps(value, pickle.HIGHEST_PROTOCOL)
PicklingError: Can't pickle <type 'function'>: attribute lookup __builtin__.function failed
有什么办法解决这个问题吗？
PickleProperty 字段需要一个可使用 Python 的 pickle 协议序列化的值（有关详细信息，请参阅 docs）：
PickleProperty: Value is a Python object (such as a list or a dict or a string) that is serializable using Python's pickle protocol; Cloud Datastore stores the pickle serialization as a blob. Unindexed by default. Optional keyword argument: compressed.
另见来自 Martijn Pieters 的 answer：
Pickle cannot handle lambdas; pickle only ever handles data, not code, and lambdas contain code. Functions can be pickled, but just like class definitions only if the function can be imported. A function defined at the module level can be imported. Pickle just stores a string in that case, the full 'path' of the function to be imported and referenced when unpickling again.
有多个选项可以使用默认值，具体取决于您的用例。