Confusing SyntaxError while entering non-indented code in interactiveinterpreter on continuation line.
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From: Colin Howell on 25 Apr 2010 16:31 [I originally sent this to python-help; the volunteer who responded thought it was OK to repost it here.] I'm sure this has been discussed somewhere before, but I can't find it in the Python issue tracker. The following behavior from the interactive interpreter is rather confusing. (I've seen this behavior both under Python 2.6.5 on 32-bit Windows XP, installed from the standard Windows binary installer available from python.org, and under Python 2.4.3 on a Fedora Core 4 32-bit x86 Linux system.) The following do-nothing code is valid Python: if True: pass pass A script file containing it will execute without error when passed to the Python interpreter from the command line, or when run from an IDLE edit window using the Run Module command (F5) key. However, if the interpreter is run in interactive mode from the command line, and I attempt to enter the same code from the interactive prompt as follows, I get a SyntaxError at the outer "pass" statement: >>> if True: .... pass .... pass File "<stdin>", line 3 pass ^ SyntaxError: invalid syntax The inner "pass" statement can be indented using spaces or with the Tab key; it doesn't matter. The problem is that I entered the outer "pass" statement on the second continuation line, rather than simply hitting return and waiting for a new primary prompt. But since the second continuation line is not indented unless you enter the indentation yourself, this is actually an easy mistake to make. One might think the parser will recognize that the inner block's indentation has been removed on the new line and that the line therefore represents a new statement. The same thing can happen in IDLE, except that IDLE automatically indents the continuation line and doesn't print a secondary prompt. But if you delete IDLE's indentation on the next continuation line and enter a new statement, you get the same SyntaxError as described above. What led me to this behavior? Example 6.1 on the following page from Dive Into Python: http://diveintopython.org/file_handling/index.html In this case, the code is a try-except statement, followed by a print. Note that the print is entered on a continuation line, which led me to carelessly try the same thing and then puzzle about it for quite a while before it finally hit me what I had been doing. I know that Dive Into Python is quite old and there have been many improvements in the language since, but I still think this gotcha needs to be looked at, at least to add a warning about it in the FAQ or tutorial, or to make the nature of the syntax error more obvious. |