file iterator - what happens here?

[Terry Reedy]

"Helmut Jarausch" <jarausch at skynet.be> wrote in message
news:3ec4a51f$0$11146$ba620e4c at reader0.news.skynet.be...
> Hi,
>
> Python's behaviour on the following code snippet
> puzzles me
>
> cmd_input= open('Tree.inp','r')
> for line in cmd_input:
>      while  line.startswith('%') : # skip comment lines
>          print "!!!",line
>          line= cmd_input.readline()
>      print "===",line,"<<<"
>      if not line: break

In general, one should not modify the iterable being iterated over by
a for statement within the for loop, certainly not the
still-to-be-iterated-over part.  Another example that does not work
right: deleting items in a list when iterating from front to back.

> So, obviously the line 'line= cmd_input.readline()'
> disturbs the for iterator completely.
> Is that a feature?

It is a fact to take cognizance of.

Terry J. Reedy