Best way to test for existence of a variable in Python

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 in a dictionary that I am using to preserve the data I have extracted.

myDict['identifier']=[VARIABLE_VAL]

So I am humming along and I get to an exception- no line in the file that has

<VARIABLENAME>theName

So 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

The reason I posted this is I was sure that I have seen somewhere a solution that looks like

if not VARIABLE_VAL:
    VARIABLE_VAL=somethingELSE

But I could not get that to work after beating my head against the monitor for thirty minutes.

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.

Answer 2

Alex's solution is correct. But just in case you do want to test if a variable exists, try:

if 'VARIABLE_VAL' in locals():
    ....