How to instantiate an object once

Question

I am instantiating this object below every time I call csv in my function. Was just wondering if there's anyway I could just instantiate the object just once? I tried to split the return csv from def csv() to another function but failed.

Code instantiating the object

def csv():

    proj = Project.Project(db_name='test', json_file="/home/qingyong/workspace/Project/src/json_files/sys_setup.json")#, _id='poc_1'
    csv = CSVDatasource(proj, "/home/qingyong/workspace/Project/src/json_files/data_setup.json")
    return csv

Test function

def test_df(csv,df)

..............

`csvObj = csv() # since you're returning csv object in the function csv()` — HaseebR7, Jun 08 '15 at 09:12
Your "code to call function" is not calling the `csv` function, it is passing the `csv` function into `test_df`, you don't show it being called. Maybe part of your confusion is that you have a name `csv` used inside the function of the same name - a bad idea. — cdarke, Jun 08 '15 at 09:26
I am missing another function to return the objects. What i want is like instantiate the objects once, have a function to return the objects and other test functions can get from it — Qing Yong, Jun 08 '15 at 09:33

score 1 · Accepted Answer · answered Jun 08 '15 at 21:08

Is your csv function actually a pytest.fixture? If so, you can change its scope to session so it will only be called once per py.test session.

@pytest.fixture(scope="session")
def csv():
    # rest of code

Of course, the returned data should be immutable so tests can't affect each other.

score 0 · Answer 2 · answered Jun 08 '15 at 09:24

0

You can use a global variable to cache the object:

_csv = None

def csv():
    global _csv
    if _csv is None:
        proj = Project.Project(db_name='test', json_file="/home/qingyong/workspace/Project/src/json_files/sys_setup.json")#, _id='poc_1'
        _csv = CSVDatasource(proj, "/home/qingyong/workspace/Project/src/json_files/data_setup.json")
    return _csv

Another option is to change the caller to cache the result of csv() in a manner similar to the snippet above.

Note that your "code to call the function" doesn't call the function, it only declares another function that apparently receives the csv function's return value. You didn't show the call that actually calls the function.

answered Jun 08 '15 at 09:24

user4815162342

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it's a bad idea to use global right? Maybe I am missing another function to return the objects. What i want is like instantiate the objects once, have a function to return the objects and other test functions can get from it – Qing Yong Jun 08 '15 at 09:32
@QingYong It's a bad idea to use a global as part of the public interface, or to pass data between module boundaries. Neither is the case here. Caching the return value of a function without exposing it in the interface is the correct (and necessary) use of global variables. This implementation "instantiates the object once", as requested. – user4815162342 Jun 08 '15 at 09:43
@downvoter It is not obvious what is wrong with the response, please elaborate. – user4815162342 Jun 08 '15 at 09:44

score 0 · Answer 3 · edited May 23 '17 at 11:51

0

You can use a decorator for this if CSVDatasource doesn't have side effects like reading the input line by line.

See Efficient way of having a function only execute once in a loop

edited May 23 '17 at 11:51

Community

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answered Jun 08 '15 at 09:53

Aaron Digulla

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score 0 · Answer 4 · answered Jun 08 '15 at 21:17

You can store the object in the function's local dictionary. And return that object if it exists, create a new one if it doesn't.

def csv():
    if not hasattr(csv, 'obj'):
        proj = Project.Project(db_name='test', json_file="/home/qingyong/workspace/Project/src/json_files/sys_setup.json")#, _id='poc_1'
        csv.obj = CSVDatasource(proj, "/home/qingyong/workspace/Project/src/json_files/data_setup.json")
    return csv.obj

How to instantiate an object once

4 Answers4