Python idiom for '... if ... else ...' expression

How to write the expression shorter:

return '%.0f' % 开发者_如何学Pythonfloat_var if float_var else float_var

or

if float_var:
    return formatted_string
else:
    return None

Thanks!

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The expression <value> if <condition> else <other_value> is pretty idiomatic already -- certainly more so than the other example, and is probably preferred whenever <value> is simple. This is Python's ternary operator, so if you were looking for something like <condition> ? <value> : <other_value>, that doesn't exist.

If computing <value> or <other_value> takes a few steps, use the longer if: ... else: ... alternative.

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I would use brackets to make the expression more readable:

return ('%.0f' % float_var) if float_var else float_var

When I first saw it I read it as

return '%.0f' % (float_var if float_var else float_var)

which would be silly. I had to try it out to make sure which way it worked.

BTW Your first example not equivalent to your second example

if float_var:
    return formatted_string
else:
    return None

This will always return either a formatted string or None. Your first example if you pass in anything that evaluates to False (False, 0, 0.0, "", [] etc) will return that unchanged, so your return type could be string, boolean, list, int, float etc. This is probably not what you want, especially if 0.0 is a legitimate value for float_var. I would change your code to:

return ('%.0f' % float_var) if isinstance(float_var, float) else None

alternatively:

try:
    return "%.0f" % float_var
except TypeError:
    return None

which will work for other integers (and longs) by converting them to float.

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• It is not clear what exactly you want to do.

  1. The most literal interpretation would have it work like this

     >>> float_var = 4.5
     >>> '%.0f' % float_var if float_var else float_var
     '5' # This is a string
     >>> float_var = 0.0
     >>> '%.0f' % float_var if float_var else float_var
     0.0 # This is a float
     

     which I am almost certain you did not want.

  2. I guess you are wanting to check for None with "if float_var"? If so, you always spell it "if foo is not None", not "if foo", the former being clearer and less bug-prone.

     If that is what you intended, I suggest you revise your model. Propagating errors by repeatedly returning None is a bad thing: it is ugly and bug-prone and non-idiomatic. Use exceptions instead.

• Shorter is not always better. Your snippets are not painfully long or unwieldly. In fact, you will want to make them a little longer if you use them to avoid a potential bug.

  • Someone may suggest abusing the short-circuiting behavior of or for this. However, this makes code harder to read and doesn't allow you to specify between None and other false values, which often leads to bugs.

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If you are using are already using v if c else u you are already using the most readable and efficient ternary operator.

There are other ways but they suffer in readability.

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float_var and "%.0f" % float_vav

Isn't it awesome?