Missing values in tuple assignment
MRAB
google at mrabarnett.plus.com
Thu Mar 19 13:37:28 EDT 2009
Albert Hopkins wrote:
> On Thu, 2009-03-19 at 11:57 -0500, Jim Garrison wrote:
>> Use case: parsing a simple config file line where lines start with a
>> keyword and have optional arguments. I want to extract the keyword and
>> then pass the rest of the line to a function to process it. An obvious
>> use of split(None,1)
>>
>> cmd,args= = line.split(None,1);
>> if cmd in self.switch: self.switch[cmd](self,args)
>> else: self.errors.append("unrecognized keyword '{0)'".format(cmd))
>>
>> Here's a test in IDLE:
>>
>> >>> a="now is the time"
>> >>> x,y=a.split(None,1)
>> >>> x
>> 'now'
>> >>> y
>> 'is the time'
>>
>> However, if the optional argument string is missing:
>>
>> >>> a="now"
>> >>> x,y=a.split(None,1)
>> Traceback (most recent call last):
>> File "<pyshell#42>", line 1, in <module>
>> x,y=a.split(None,1)
>> ValueError: need more than 1 value to unpack
>>
>> I understand the problem is not with split() but with the assignment
>> to a tuple. Is there a way to get the assignment to default the
>> missing values to None?
>
> why not do this?
> >>> a= 'now'
> >>> z = a.split(None, 1)
> >>> x = z[0]
> >>> y = z[1] if len(z) == 2 else None
>
A 1-line solution (not necessarily recommended) is:
x, y = (a.split(None, 1) + [None])[ : 2]
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