Cannot change exchange amount about brightway2 HOT 5 CLOSED

Original comment by Benjamin W. Portner (Bitbucket: pommespapst, GitHub: pommespapst).

Better-formatted code:

#!python

oActivity = bw.Database('some_db').search("some_activity")[0] 
oExchange = list(oActivity.exchanges())[1] 
oExchange["amount"]
# 0
oExchange["amount"] = 1 
oExchange["amount"] 
# still 0

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Original comment by Chris Mutel (Bitbucket: cmutel, GitHub: cmutel).

Sorry, that is frustrating! I can't reproduce this:

#!python

In [6]: db = Database("ecoinvent 3.5 cutoff")

In [7]: exc = list(db.random().exchanges())[0]

In [8]: exc['amount']
Out[8]: 0.000277

In [9]: exc['amount'] = 1

In [10]: exc['amount']
Out[10]: 1

Some things to try:

1. Make sure your version of bw2data is up to date - latest is 3.4.4.
2. Make sure you are getting an actual activity and exchange (not sure how this wouldn't be the case, but check anyway)
3. If you are loading the exchange in the second half, note that your changes will only be written to the database if you call oExchange.save() after you make your modifications.

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Original comment by Benjamin W. Portner (Bitbucket: pommespapst, GitHub: pommespapst).

Looks like the error occurs if the handle to the exchange is not saved:

#!python

oAct = bw.Database('ecoinvent 3.5 APOS').random()

# does not work
list(oAct.exchanges())[0]["amount"]
list(oAct.exchanges())[0]["amount"] = 3
list(oAct.exchanges())[0]["amount"]

# works
oExc = list(oAct.exchanges())[0]
oExc["amount"]
oExc["amount"] = 3
oExc["amount"]

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Original comment by Chris Mutel (Bitbucket: cmutel, GitHub: cmutel).

There are two important details here:

1. list(oAct.exchanges())[0]["amount"] will retrieve the exchanges from the database each time, and there is no guarantee that they are in the same order (by default database queries have no defined order unless you specify one). I am not sure if Brightway gives a default order - by guess is yes - but in general SQL queries don't unless you specify ORDER BY.
2. list(oAct.exchanges())[0]["amount"] also won't work the second time because you are recreating the object each time. The changes you make in the first instance are not saved, and so disappear. It is the same as:

#!python

In [1]: class A:
   ...:     pass
   ...:

In [2]: a = A()

In [3]: a.foo = "bar"

In [4]: a = A()

In [5]: a.foo
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-031223489f27> in <module>()
----> 1 a.foo

AttributeError: 'A' object has no attribute 'foo'

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Original comment by Benjamin W. Portner (Bitbucket: pommespapst, GitHub: pommespapst).

Thanks for the clarification!

Concerning the order of exchanges: it is changing every time indeed. Thanks for the hint!

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