问题
Sometimes in Python scripts I see lines like:
cmd = "%s/%s_tb -cm cond+line+fsm -ucli -do \"%s\""
Where is the %s in the above line substituted? Does Python have some stack of strings and it pops them and replaces %s?
回答1:
That would be later used in something like:
print cmd % ('foo','boo','bar')
What you're seeing is just a string assignment with fields in it which will later be filled in.
回答2:
Basics of python string formatting
Not a specific answer to your line of code, but since you said you're new to python I thought I'd use this as an example to share some joy ;)
Simple Example Inline With a List:
>>> print '%s %s %s'%('python','is','fun')
python is fun
Simple Example Using a Dictionary:
>>> print '%(language)s has %(number)03d quote types.' % \
... {"language": "Python", "number": 2}
Python has 002 quote types
When in doubt, check the python official docs - http://docs.python.org/library/stdtypes.html#string-formatting
回答3:
It's being used for string interpolation. The %s is replaced by a string. You use the modulo operator (%) to do string interpolation. The string will be on the left side, the values to substitute for the various %s are on the right, in a tuple.
>>> s = '%s and %s'
>>> s % ('cats', 'dogs' )
<<< 'cats and dogs'
If you have just a single character you can forget the tuple.
>>> s = '%s!!!'
>>> s % 'what'
<<< 'what!!!'
In newer versions of python the recommend way is to use the format method of the string type:
>>> '{0} {1}'.format('Hey', 'Hey')
<<< 'Hey Hey'
来源：https://stackoverflow.com/questions/5954260/default-substituting-s-in-python-scripts