New submission from Craig McQueen <[email protected]>:
I have just been trying to figure out how string interpolation works for "%s",
when Unicode strings are involved. It seems it's a bit complicated, but the
Python documentation doesn't really describe it. It just says %s "converts any
Python object using str()".
Here is what I have found (I think), and it could be worth improving the
documentation of this somehow.
Example 1:
"%s" % test_object
>From what I can tell, in this case:
1. test_object.__str__() is called.
2. If test_object.__str__() returns a string object, then that is substituted.
3. If test_object.__str__() returns a Unicode object (for some reason), then
test_object.__unicode__() is called, then _that_ is substituted instead. The
output string is turned into Unicode. This behaviour is surprising.
[Note that the call to test_object.__str__() is not the same as
str(test_object), because the former can return a Unicode object without
causing an error, while the latter, if it gets a Unicode object, will then try
to encode('ascii') to a string, possibly generating a UnicodeEncodeError
exception.]
Example 2:
u"%s" % test_object
In this case:
1. test_object.__unicode__() is called, if it exists, and the result is
substituted. The output string is Unicode.
2. If test_object.__unicode__() doesn't exist, then test_object.__str__() is
called instead, converted to Unicode, and substituted. The output string is
Unicode.
Example 3:
"%s %s" % (u'unicode', test_object)
In this case:
1. The first substitution causes the output string to be Unicode.
2. It seems that (1) causes the second substitution to follow the same rules as
Example 2. This is a little surprising.
----------
assignee: d...@python
components: Documentation
messages: 109516
nosy: cmcqueen1975, d...@python
priority: normal
severity: normal
status: open
title: Improve docs for string interpolation "%s" re Unicode strings
versions: Python 2.7
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Python tracker <[email protected]>
<http://bugs.python.org/issue9196>
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