[Python] string.find() aways evaluates to true

I have a small script that reads a file. After reading a line I'm trying to figure out if the particular line has a specific text in it. I do so like this:

for line in file:
line = line.lower()

if line.find('my string'):
print ('found my string in the file')


I find that line.find always evaluates to true. When I do this

for line in file:
line = line.lower()

if 'one big line'.find('my string'):
print ('found my string in the file')


it evaluates to false, as it's supposed to do.

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