Failed to catch syntax error python
Question:
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)]
Answers:
You can only catch SyntaxError
if it’s thrown out of an eval
, exec
, or import
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.
SyntaxError
s 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).
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)]
You can only catch SyntaxError
if it’s thrown out of an eval
, exec
, or import
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.
SyntaxError
s 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).