Python Syntax Errors with Python Anywhere

Question

I'm using Python with DataCamp and Python Anywhere, and they seem to disagree on what a syntax error is. I've recently just started, so I tried this line of code:

n = 5
while n > 0:
   print (n)
   n = n - 1
print ('Blastoff!')

It runs like it's supposed on DataCamp, but with Python Anywhere, I get the following error:

  File "<stdin>", line 5
    print ("Blastoff!")
        ^
SyntaxError: invalid syntax

I don't know what it's referencing or trying to tell me. The error message is unhelpful, and I don't know why I'm getting two different evaluations here.

Answer 1

When pasting into interactive interpreter, you must have an empty line after a block statement, before the next statement. Here's the output from the Python Anywhere interpreter embedded on http://www.python.org:

Python 3.6.0 (default, Jan 13 2017, 00:00:00) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> n = 5
>>> while n > 0:
...    print (n)
...    n = n - 1
... print ('Blastoff!')
  File "<stdin>", line 4
    print ('Blastoff!')
        ^
SyntaxError: invalid syntax
>>> 

Writing anything in the first column to ... will cause this SyntaxError, even though legal in a source file. This is because all compound statements are passed into exec(compile(... 'single')) when completed; and the python REPL is being a bit stupid here, thinking that it was just one statement, when it is in fact while followed by a print.

Hitting enter so that the prompt returns to >>> before print will fix the problem in interactive interpreters:

Python 3.6.0 (default, Jan 13 2017, 00:00:00) 
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> n = 5
>>> while n > 0:
...    print (n)
...    n = n - 1
... 
5
4
3
2
1
>>> print ('Blastoff!')
Blastoff!
>>> 

But notice that the while loop now runs as soon as the compound statement is terminated, i.e. before the >>> prompt shows again.

There are other shells besides the standard Python REPL. One popular, ipython, has a console shell that will recognize copy-pasted content and run this one correctly:

% ipython
Python 3.5.3 (default, Jan 19 2017, 14:11:04) 
Type 'copyright', 'credits' or 'license' for more information
IPython 6.1.0 -- An enhanced Interactive Python. Type '?' for help.

In [1]: n = 5
   ...: while n > 0:
   ...:    print (n)
   ...:    n = n - 1
   ...: print ('Blastoff!')
   ...: 
5
4
3
2
1
Blastoff!

In [2]: 

Answer 2

Both PythonAnywhere and the shell will treat anything within the ... as part of the first statement, meaning that anything after a if or with or while or for that starts with 3 dots is supposed to be executed when the first statement is evaluated.

If you had an if statement, any code entered while the ... is there will be executed when the if statement is evaluated.