Failed to catch syntax error python [duplicate]

问题:

This question already has an answer here:

• SyntaxError inconsistency in Python? 2 answers

try:     x===x except SyntaxError:     print "You cannot do that" 

outputs

    x===x        ^ SyntaxError: invalid syntax 

this does not catch it either

try:     x===x except:     print "You cannot do that" 

Other errors like NameError, ValueError, are catchable.

Thoughts?

System specs:

import sys print(sys.version) 

-> 2.7.5 (default, Mar 9 2014, 22:15:05) [GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)]

回答1:

You can only catch SyntaxError if it's thrown out of an eval or exec operation.

>>> try: ...    eval('x === x') ... except SyntaxError: ...    print "You cannot do that" ...  You cannot do that 

This is because, normally, the interpreter parses the entire file before executing any of it, so it detects the syntax error before the try statement is executed. If you use eval or its friends to cause more code to be parsed during the execution of the program, though, then you can catch it.

I'm pretty sure this is in the official manual somewhere, but I can't find it right now.

回答2:

SyntaxErrors get raised when the file/code is parsed, not when that line of code is executed. The reason for this is simple -- If the syntax is wrong at a single point in the code, the parser can't continue so all code after that line is un-parseable.

In other words, you can only catch syntax errors when python is trying to parse something. This includes exec, eval, import:

>>> try: ...    import junk ... except SyntaxError: ...    print "Woo" ...  Woo 

and various things regarding ast and the like.

Note that the python tutorial even distinguishes between SyntaxError and other exceptions although the distinction isn't as clear as the tutorial makes it seem (since you can in fact catch SyntaxError if you know when they get raised).

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