s='s=%r;print(s%%s)';print(s%s)
I understand % is to replace something in a string by s (but actually who to replace?)
Maybe more intriguing is, why the print(s%%s) become print(s%s) automatically after %s is replaced by s itself?
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bozeng
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i changed the problem so now it asks about %%s – bozeng Jan 26 '15 at 06:38
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1`%%` is how you write a literal `%` in a format string. – Dan D. Jan 26 '15 at 06:39
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no. if i let a="print(s%%s)" – bozeng Jan 26 '15 at 06:42
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1Consider `"%s%%s" % 'follow me '` => `'follow me %s'`. – Dan D. Jan 26 '15 at 06:43
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Ok i got it. %% only reduced to % when you actually use % for this string........... – bozeng Jan 26 '15 at 06:46
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but it will not happen if you do not use replacement. btw who invented % as replacement. is this Python unique? or C also use it? – bozeng Jan 26 '15 at 06:47
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C has it in the format strings passed to `printf`. Although some of the letters are used differently. – Dan D. Jan 26 '15 at 06:49
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but not with string%something, it is % inside string, and something as arguments to printf right? – bozeng Jan 26 '15 at 06:50
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bozeng is asking about % the (Python) ***format operator***, as in `formatstring % formatitems`, not the use of % inside the format-string fields: "%s %5.2f" – smci Jan 26 '15 at 07:23
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The "%%" you see in that code is a "conversion specifier" for the older printf-style of string formatting.
Most conversion specifiers tell Python how to convert an argument that is passed into the % format operator (for instance, "%d" says to convert the next argument to a decimal integer before inserting it into the string).
"%%" is different, because it directly converts to a single "%" character without consuming an argument. This conversion is needed in the format string specification, since otherwise any "%" would be taken as the first part of some other code and there would be no easy way to produce a string containing a percent sign.
The code you show is a quine (a program that produces its own code as its output). When it runs print(s%s), it does a string formatting operation where both the format string, and the single argument are the same string, s.
The "%r" in the string is a conversion specifier that does a repr of its argument. repr on a string produces the string with quotes around it. This is where the quoted string comes from in the output.
The "%%" produces the % operator that appears between the two s's in the print call. If only one "%" was included in s, you'd get an error about the formatting operation expecting a second argument (since %s is another conversion specifier).
Blckknght
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I agree with you. But to note if you use "%%" directly, and print it , it is a double percent. E.g: print("%%") and see the output – bozeng Jan 26 '15 at 07:34
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It means its behavior is not consistent cross at least print and print with format – bozeng Jan 26 '15 at 07:34
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Your right, that by itself, `"%%"` is just a string with two percent signs in it. It's only when you use that as a format string that the two characters turn into one. Try `"%%" % ()`, to format the string with no arguments and you'll see it happen. – Blckknght Jan 26 '15 at 07:36
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print '% %s' % '' #wrong
print '%% %s' % '' #correct and print '% '
Think about \\ and \.
John Hua
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My point is about double %. As more above academic answers had mentioned, any string need to be persentaged mustn't contain a single %. JUST use double % instead. – John Hua Jan 26 '15 at 12:05