Best way to test for existence of a variable in Python [duplicate]

Question

I am reading a million plus files to scrape out some data. The files are generally pretty uniform but there are occasional problems where something that I expected to find is not present.

For example, I expect some sgml code to identify a value I need

for data_line in temp  #temp is a list of lines from a file
    if <VARIABLENAME> in data_line:
        VARIABLE_VAL=data_line.split('>')[-1]

Later on I use VARIABLE_VAL. But I sometimes get an exception: no line in the file that has

<VARIABLENAME>theName

To handle this I have added this line after all the lines have been processed:

try:
    if VARIABLE_VAL:
        pass
except NameError:
    VARIABLE_VAL=somethingELSE

I have seen somewhere (but I can't find it anymore) a solution that looks like

if not VARIABLE_VAL:
    VARIABLE_VAL=somethingELSE

Any help would be appreciated

Answer 1

Just initialize your variable to its default value before the loop:

VARIABLE_VAL = somethingELSE
for dataline in temp: ...

this way, VARIABLE_VAL will keep its initial, default value unless bound to something else within the loop, and you need no weird testing whatsoever to ensure that.