CODEWITHSUNDEEP
pythonscope
flamey
flamey

Reputation: 2379

How do I live with variables not being local in for loop in Python

I'm new to Python (previously did some Perl). I'm a bit confused about how to live without "for" loop's variable not being local to the loop :) The question is in general how to use it without it biting me at some point, and in my example below in particular (now TestCase "keeps" notes from previous execution) -- or am I trying to do something silly there?

I guess in that for loop I could somehow undefine Name and TestCase. But what if it was a rather complex block of code, with continue-s etc.. how would I ensure Name and TestCase are always "clean" in the beginning on the loop?

class Result():
    def __init__(self, result, notes=[]):
        self.Res = result
        self.Notes  = notes   # List of strings
    # ...

def ExecTest1(p):
    ret = Result(PASS)
    # ...
    ret.FAIL('some note') # appends string as list item to Notes
    return ret

def ExecTest1(p):
    ret = Result(PASS)
    return ret

for Name, TestCase in {
        'Negative test': ExecTest1( param ),
        'Positive test': ExecTest1( another_param ),
    }.iteritems():

    print Name, TestCase.Res   # string
    print TestCase.Notes       # list

Upvotes: 0

Views: 96

Answers (1)

Duncan
Duncan

Reputation: 95682

What you are actually seeing here is a feature of mutable default arguments, nothing to do with the scope of a for loop.

class Result():
    def __init__(self, result, notes=[]):
        self.Res = result
        self.Notes  = notes   # List of strings
    # ...

Here the default argument for notes is the same list used every time you don't specify another list. When you append a value to that list the value will still be there in every other instance of Result.

What you want to do instead is:

class Result():
    def __init__(self, result, notes=None):
        self.Res = result
        self.Notes  = notes if notes is not None else []  # List of strings
    # ...

as that will create a new empty list whenever the default None value is used for notes.

Default arguments for functions are computed when the function is defined. i.e. when the def statement is executed. The resulting object is saved as part of the function and is reused whenever the function is called.

You can read more about this in the Python FAQ

Upvotes: 1

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