Pymongo BSON Binary是否保存和检索? [英] Pymongo BSON Binary save and retrieve?

问题描述

我正在与MongoDB一起使用Python尝试紧密保存浮点数组.

I'm working in Python with MongoDB trying to save an array of floats tightly.

我可以正确创建和存储*

但是我无法以可用格式检索数据.

>>> import random, array, pymongo
>>> from bson.binary import Binary as BsonBinary
>>> con = pymongo.Connection('localhost', 27017)
>>> mm = con['testDatabase']
>>> vals = [random.random() *100 for x in range(1, 5)]
>>> vals
[2.9962593, 64.5582810776, 32.3781311717, 82.0606953423]
>>> varray = array.array('f', vals)
>>> varray
array('f', [2.9962593, 64.5582810776, 32.3781311717, 82.0606953423])
>>> vstring = varray.tostring()
>>> vstring
'\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B'
>>> vbson = BsonBinary(vstring, 5)
>>> vbson
Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)
>>> doc1 = { 'something': 1 , 'else' : vbson}
>>> doc1
{'something': 1, 'else': Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)}
>>> mm.test1.insert(doc1)
ObjectID('530f7af1d809d80d3db1f635')
>>> gotdoc = mm.test1.find_one()
>>> gotdoc
{u'_id': ObjectId('530f7af1d809d80d3db1f635'), u'something': 3, u'else': Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)}
>>> gotfield = gotdoc['else']
>>> gotfield
Binary('\xb7\xc2?@\xd7\x1d\x81B5\x83\x01B\x13\x1f\xa4B', 5)
>>> from bson import BSON
>>> BSON.decode(gotfield)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unbound method decode() must be called with BSON instance as first argument (got Binary instance instead)
>>> gotfield.decode()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
UnicodeDecodeError: 'ascii' codec can't decode byte 0xb7 in position 0: ordinal not in range(128)
>>>

一旦我取回Python字符串，就可以取回我的随机浮点数数组.但是如何?

推荐答案

让我们解决以下错误:

1. 出现第一个错误仅仅是因为您需要一个实际的BSON对象.请注意，您从未编码任何数据-创建bson.binary.Binary对象并不意味着调用BSON.encode().

1. The first error appears simply because you need an actual BSON object. Note, that you have never encoded any data - creating bson.binary.Binary object does not mean invoking BSON.encode().

这就是PyMongo欺骗您的地方. bson.binary.Binary是运行时修补的str或bytes实例(请参阅源文件).这就是为什么您会收到第二个错误的原因:您实际上调用的是str.decode()，而不是BSON.decode().因此，gotfield 包含您最初存储的随机浮点数据，但是对象本身绑定了一些不同的方法(例如repr()).

And that is where PyMongo cheats you a bit. The bson.binary.Binary is a runtime-patched str or bytes instance (see source). That is why you get the second error: what you call is actually str.decode(), not BSON.decode(). So, gotfield contains the random float data you've stored initially, but the object itself has some different methods (e.g. repr()) bound to it.

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