Python Try-Except inside of Function

Question

I've got a pretty good understanding of python's try-except clause, but I'm encountering problems when trying to put it inside of a function.

>>> def tryAppend(child, parent):
...     try:
...             parent.append(child)
...     except NameError:
...             print "WRONG NAME"
>>> var1 = []
>>> var2 = 'test2'
>>> tryAppend(var2, var1)  #works, no error
>>> tryAppend(foo, var1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'foo' is not defined

it is almost like python doesn't see the try: statement. Any help is appreciated.

Solution 1

This has nothing to do with your exception handler. The error you are seeing is because "foo" is not defined anywhere.

Solution 2

The name error is happening before it ever gets into tryAppend. It evaluates the value of foo when trying to pass it to the function. This works:

def tryAppend(child, parent):
    parent.append(child)

var1 = []
try:
    tryAppend(foo, var1)
except NameError:
    print 'WRONG NAME'

Solution 3

The NameError is being thrown when the name 'foo' is evaluated, which is before entering the function. Therefore the try/except within the function isn't relevant.

Solution 4

For someone who is looking for how to use try except construction inside of the function. I am not sure whether it is a good programming style, but it works.

You can put string arguments to the function. It will be evaluated correctly and then you can use exec inside of the function:

def tryAppend(child, parent):
    try:
        script = parent + '.append(' + child + ')'
        exec script
        return parent
    except NameError:
        print "WRONG NAME"
var1 = []
var2 = 'test2'
tryAppend('var2', 'var1')
tryAppend('foo', 'var1')

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