Problem redefining __getitem__ for str subclass

[tsm8015]

I do not think I am understanding how to redefine the getitem function
for string. Why does the following not work:

class NStr(str):
def __getitem__(self,idx):
print "NStr:getitem",idx,type(idx)
return str.__getitem__(self,idx)

s=NStr("abcde")

print s[1]
print s[1:4:2]
print s[1:2]

if I run this program (python 2.5; Mac OSX) i get:

$ python strProb.py
NStr:getitem 1 <type 'int'>
b
NStr:getitem slice(1, 4, 2) <type 'slice'>
bd
b

ie the last statement (s[1:2]) with a simple slice does not call the
new __getitem__???? What am I missing.

Thanks

tom

 

[Michel Claveau]

Hi!

Same result, with Python 2.5.1 on win-XP;
I have compassion with you.

 

[Carsten Haese]

On 21 Apr 2007 20:36:13 -0700, tsm8015 wrote

I do not think I am understanding how to redefine the getitem
function for string. Why does the following not work:

class NStr(str):
def __getitem__(self,idx):
print "NStr:getitem",idx,type(idx)
return str.__getitem__(self,idx)

s=NStr("abcde")

print s[1]
print s[1:4:2]
print s[1:2]

if I run this program (python 2.5; Mac OSX) i get:

$ python strProb.py
NStr:getitem 1 <type 'int'>
b
NStr:getitem slice(1, 4, 2) <type 'slice'>
bd
b

ie the last statement (s[1:2]) with a simple slice does not call the
new __getitem__???? What am I missing.

You're overriding __getitem__ just fine, but if your object has a __getslice__
method, __getslice__ will be called instead of __getitem__ to fetch simple
slices. Since you are deriving from str which does implement __getslice__,
you'll need to override __getslice__, too.

-Carsten

 

[Alex Martelli]

I do not think I am understanding how to redefine the getitem function
for string. Why does the following not work:

class NStr(str):
def __getitem__(self,idx):
print "NStr:getitem",idx,type(idx)
return str.__getitem__(self,idx)

s=NStr("abcde")

print s[1]
print s[1:4:2]
print s[1:2]

if I run this program (python 2.5; Mac OSX) i get:

$ python strProb.py
NStr:getitem 1 <type 'int'>
b
NStr:getitem slice(1, 4, 2) <type 'slice'>
bd
b

ie the last statement (s[1:2]) with a simple slice does not call the
new __getitem__???? What am I missing.

For backwards compatibility, slicing without a step on some ancient
built-in types (including strings, but also lists) goes to __getslice__
(an otherwise obsolete method) instead of __getitem__. Simply, override
__getslice__ by properly delegating from it to __getitem__.


Alex