Default argument to __init__

[netvaibhav]

Hi All:

Here's a piece of Python code and it's output. The output that Python
shows is not as per my expectation. Hope someone can explain to me this
behaviour:

[code]
class MyClass:
def __init__(self, myarr=[]):
self.myarr = myarr

myobj1 = MyClass()
myobj2 = MyClass()

myobj1.myarr += [1,2,3]

myobj2.myarr += [4,5,6]

print myobj1.myarr
print myobj2.myarr
[/code]

The output is:
[1, 2, 3, 4, 5, 6]
[1, 2, 3, 4, 5, 6]

Why do myobj1.myarr and myobj2.myarr point to the same list? The
default value to __init__ for the myarr argument is [], so I expect
that every time an object of MyClass is created, a new empty list is
created and assigned to myarr, but it seems that the same empty list
object is assigned to myarr on every invocation of MyClass.__init__

It this behaviour by design? If so, what is the reason, as the
behaviour I expect seems pretty logical.

Thanks.

Vaibhav

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Re: Scanning a file

[netvaibhav]

I think implementing a finite state automaton would be a good (best?)
solution. I have drawn a FSM for you (try viewing the following in
fixed width font). Just increment the count when you reach state 5.

<---|
   ||
0  0   |  1  0  |0
 -->[1]--->[2]--->[3]--->[4]--->[5]-|
 ^ ||  ^ | |  |
1| |<---|  | | |1 |1
 |_|1  |_| |  |
 ^ 0   |  |
 |-|<-|

If you don't understand FSM's, try getting a book on computational
theory (the book by Hopcroft & Ullman is great.)

Here you don't have special cases whether reading in blocks or reading
whole at once (as you only need one byte at a time).

Vaibhav

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Re: Scanning a file

[netvaibhav]

Steve Holden wrote:
> Indeed, but reading one byte at a time is about the slowest way to
> process a file, in Python or any other language, because it fails to
> amortize the overhead cost of function calls over many characters.
>
> Buffering wasn't invented because early programmers had nothing better
> to occupy their minds, remember :-)

Buffer, and then read one byte at a time from the buffer.

Vaibhav

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Re: altering an object as you iterate over it?

[netvaibhav]

bruno at modulix wrote:
> fin = open(path, 'r')
> fout = open(temp, 'w')
> for line in fin:
>   if line.strip():
> fout.write(line)
> fin.close()
> fout.close()
>
> then delete path and rename temp, and you're done. And yes, this is
> actually the canonical way to do this !-)

What if there's a hard link to path?


-- 
Vaibhav

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