Why do people write #!/usr/bin/env python on the first line of a Python script?

Question

It seems to me like the files run the same without that line.

Answer 1

It just specifies what interpreter you want to use. To understand this, create a file through terminal by doing touch test.py, then type into that file the following:

#!/usr/bin/env python3
print "test"

and do chmod +x test.py to make your script executable. After this when you do ./test.py you should get an error saying:

  File "./test.py", line 2
    print "test"
               ^
SyntaxError: Missing parentheses in call to 'print'

because python3 doesn't supprt the print operator.

Now go ahead and change the first line of your code to:

#!/usr/bin/env python2

and it'll work, printing test to stdout, because python2 supports the print operator. So, now you've learned how to switch between script interpreters.