I learned that a script run by crontab can't have a relative path.
This fix was here: http://stackoverflow.com/a/10422444
The following example opens my_file.txt in a path relative to your script:
dir_path = os.path.dirname(os.path.abspath(__file__))
with open (os.path.join(dir_path, 'my_file.txt'), "r") as myfile:
mytext = myfile.read()