Update main.py

[alanyee]

-Avoid the use of dangerous default values -Send print statement into stderr -Use sys.exit instead of exit

[coveralls]

Coverage decreased (-0.4%) to 99.59% when pulling 40b493eb9be4dba3c9f45e61706070bb2f5ec5c0 on alanyee:patch-1 into 78f1a384eddef16906732d8db66deea6d37049b7 on noahmorrison:master.

[noahmorrison]

merged