Python: try statement in a single line

This is terribly hackish, but I've used it at the prompt when I wanted to write up a sequence of actions for debugging:

exec "try: some_problematic_thing()\nexcept: problem=sys.exc_info()"
print "The problem is %s" % problem[1]

For the most part, I'm not at all bothered by the no-single-line-try-except restriction, but when I'm just experimenting and I want readline to recall a whole chunk of code at once in the interactive interpreter so that I can adjust it somehow, this little trick comes in handy.

For the actual purpose you are trying to accomplish, you might try locals().get('c', b); ideally it would be better to use a real dictionary instead of the local context, or just assign c to None before running whatever may-or-may-not set it.

There is no way to compress a try/except block onto a single line in Python.

Also, it is a bad thing not to know whether a variable exists in Python, like you would in some other dynamic languages. The safer way (and the prevailing style) is to set all variables to something. If they might not get set, set them to None first (or 0 or '' or something if it is more applicable.)

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If you do assign all the names you are interested in first, you do have options.

• The best option is an if statement.

  c = None
  b = [1, 2]
  
  if c is None:
      a = b
  else:
      a = c
  

• The one-liner option is a conditional expression.

  c = None
  b = [1, 2]
  a = c if c is not None else b
  

• Some people abuse the short-circuiting behavior of or to do this. This is error prone, so I never use it.

  c = None
  b = [1, 2]
  a = c or b
  

  Consider the following case:

  c = []
  b = [1, 2]
  a = c or b
  

  In this case, a probably should be [], but it is [1, 2] because [] is false in a boolean context. Because there are lots of values that can be false, I don't use the or trick. (This is the same problem people run into when they say if foo: when they mean if foo is not None:.)

In python3 you can use contextlib.suppress:

from contextlib import suppress

d = {}
with suppress(KeyError): d['foo']