Strange (?) list comprehension behavior

[George Henry]

I am a Python newbie, using IDLE, writing a lexical analyzer for a small
expression language in Python 2.2.3.

class PSILex:
ops = ["~", "!", "@", ... "|", "||", ... ]
...
def __findAtom(self):
result = ""
ops = [op for op in PSILex.ops if self.text.startswith(op)]

.... results in a traceback and "TypeError: expected a character buffer
object." In trying to figure out what was going on, in the Python Shell I
subsequently tried

[op for op in PSILex.ops if "|| hello".startswith(op)]

with the same result. Printing type(self.text) immediately prior ot the
above line proves that that attribute's type has not been compromised, so
the problem appears to be with 'op'.

[op for op in PSILex.ops if op.startswith("|| hello")]

yields "AttributeError: 'int' object has no attribute 'startswith',"
however:

[type(op) for op in PSILex.ops]

produces a list of <type 'str'> objects, as would be expected. So, what is
op? Is it a string, an int, or something else? It appears that the
interpreter may be confused. I persuaded the interpreter to cooperate by
injecting an explicit evaluation of type(op):

ops = [op for op in PSILex.ops if type(op) == type("") and \
self.text.startswith(op)]

and this does what I want, and what I would expect without 'type(op) ==
type("") and '.

Can someone explain what is happening here? Or should I send my code to
ww.python.org as a "defect report?"

Regards,
George

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[Andrew Dalke]

George Henry:

[op for op in PSILex.ops if op.startswith("|| hello")]

yields "AttributeError: 'int' object has no attribute 'startswith',"
however:

[type(op) for op in PSILex.ops]

produces a list of <type 'str'> objects, as would be expected. So, what is
op? Is it a string, an int, or something else?

Can you copy&paste what you did, rather than interpret the output?
There's no way to get both of these errors that I know, and I expect
it to be more likely that you didn't see a <type 'int'> somewhere in
the output rather than a bug in Python.

You can also try

[type(op) for op in PSILex.ops if type(op) != type("")]

Andrew
(e-mail address removed)

 

[Peter Hansen]

class PSILex:
ops = ["~", "!", "@", ... "|", "||", ... ]

What are those ...'s ? You may have left out critical
data for us.

...
def __findAtom(self):
result = ""
ops = [op for op in PSILex.ops if self.text.startswith(op)]

... results in a traceback and "TypeError: expected a character buffer
object." In trying to figure out what was going on, in the Python Shell I
subsequently tried

Please post the precise traceback, cut and pasted. If it really said
that, and your subsequent analysis is valid, I think one of your "ops"
is not a string...

 

[Peter Hansen]

George Henry wrote: [a question]

Please post the precise traceback, cut and pasted. If it really said
that, and your subsequent analysis is valid, I think one of your "ops"
is not a string...

Cancel that... I didn't see your subsequent followup before I posted,
because it was not linked to the previous thread. You already found
the problem...

-Peter