[Python-bugs-list] [ python-Bugs-524804 ] breaking file iter loop leaves file in stale state

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Bugs item #524804, was opened at 2002-03-02 10:44
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Category: Python Library
Group: Python 2.2
>Status: Closed
>Resolution: Wont Fix
Priority: 5
Submitted By: Just van Rossum (jvr)
Assigned to: Guido van Rossum (gvanrossum)
Summary: breaking file iter loop leaves file in stale state

Initial Comment:
Given a file created with this snippet:

  >>> f = open("tmp.txt", "w")
  >>> for i in range(10000):
  ...     f.write("%s\n" % i)
  ... 
  >>> f.close()

Iterating over a file multiple times has unexpected 
behavior:

  >>> f = open("tmp.txt")
  >>> for line in f:
  ...     print line.strip()
  ...     break
  ... 
  0
  >>> for line in f:
  ...     print line.strip()
  ...     break
  ... 
  1861
  >>> 

I expected the last output line to be 1 instead of 
1861.

While I understand the cause (xreadlines being 
used by the
file iterator, it reads a big chunk ahead, causing 
the actual
filepos to be out of sync), this seems to be an 
undocumented
gotcha. The docs say this:

  [ ... ] Each iteration returns the same result as
  file.readline(), and iteration ends when the 
readline()
  method returns an empty string. 

That is true within one for loop, but not when you 
break out
of the loop and start another one, which I think is a 
valid
idiom.

Another example of breakage:

  f = open(...)
  for line in f:
      if somecondition(line):
	  break
      ...
  
  data = f.read()  # read rest in one slurp

The fundamental problem IMO is that the file 
iterator stacks
*another* state on top of an already stateful object. 
In a
sense a file object is already an iterator. The two 
states get
out of sync, causing confusing semantics, to say 
the least.
The current behavior exposes an implementation 
detail that
should be hidden.

I understand that speed is a major issue here, so 
a solution
might not be simple.

Here's a report from an actual user:
http://groups.google.com/groups?hl=en&selm=
owen-
0B3ECB.10234615022002%40nntp2.u.washingto
n.edu
The rest of the thread suggests possible 
solutions.

Here's what I *think* should happen (but: I'm 
hardly aware
of both the fileobject and xreadline innards) is this:
xreadlines should be merged with the file object. 
The buffer
that xreadlines implements should be *the* buffer 
for the
file object, and *all* read methods should use *
that* buffer
and the according filepos.

Maybe files should grow a .next() method, so iter(f) 
can return
f itself. .next() and .readline() are then 100% 
equivalent.


----------------------------------------------------------------------

>Comment By: Guido van Rossum (gvanrossum)
Date: 2002-03-22 22:40

Message:
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Closing as won't fix.

----------------------------------------------------------------------

Comment By: Guido van Rossum (gvanrossum)
Date: 2002-03-19 22:43

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There are two forces at work here.

You want the most common case (a single "for line in file"
that consumes the whole file) to run blindingly fast.

And you want full generality, basically equating next() with
readline() (except raising StopIteration on EOF).

Unfortunately, the only way to go blindingly fast is to do
aggressive buffering, and that's what xreadlines does. Until
we rewrite the entire I/O system so we have total control
over buffering ourselves, it's not easy to mix xreadlines
with other operations (readline, seek, tell).

We could make the default file iterator use readline, but
then it would become much slower, and we'd have to teach
people about xreadlines if they want speed. Or we could use
the current solution, where speed is the default, and you
have to be more careful when you use an unusual coding
style, like breaking out of the for loop and continuing in a
second for loop.

I'm not sure which requirement is more common (speed, or
generality), but since looping over all the lines of a (big)
file is such a common pattern, I would bet the speed is the
more important of the two.

In the past we've had a lot of flak about the slowness of
the general solution of looping over all lines in a file;
the xreadlines-based iterator is much faster, and I am
reluctant to change this back in Python 2.3; I'd rather
document it carefully (after all, "for line in lines" is a
new construct in Python 2.2, and people have to be told
about it; we might as well tell them about the limitations
and how to work around them).

----------------------------------------------------------------------

Comment By: Matthias Urlichs (smurf)
Date: 2002-03-19 12:37

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I'm in favor for the "let files be their own iterator and set .next equal to .readline" solution. The above example can _really_ bite you.

The current xreadlinesmodule.c could be converted to a standalone module, if it really is necessary to optimize.

The trivial solution for this problem is to change CHUNKSIZE (in Modules/xreadlinesmodule) to 1. Or, even better, to convert it into an instance variable, so you can do this:

f=open(...)
fi=f.iter(chunk=2000)
for line in fi:
    ...

if you want speed, or just write

for line in f:

(which internally converts to f.iter(chunk=1))

if you want safety.

I'm not too firm with Python C interfacing, otherwise I'd write a patch... any takers?

----------------------------------------------------------------------

Comment By: Jason Orendorff (jorend)
Date: 2002-03-08 04:35

Message:
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Tim wrote: "I'm sure Guido was aware of this."

    ...Wow, really?  That kind of puts a damper on
       my theory.  I guess it can't be a bug after
       all.  :)

Tim wrote: "Making the simplest-to-spell idiom as fast as 
possible was a deliberate decision at the time."

    ...*That* I believe.


----------------------------------------------------------------------

Comment By: Just van Rossum (jvr)
Date: 2002-03-08 04:08

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At the cost of, what, sensible, predictable semantics?
- fast is better than slow
- but slow is better than unpredictable
Or something...

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Comment By: Tim Peters (tim_one)
Date: 2002-03-07 20:16

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I'm sure Guido was aware of this.  Making the simplest-to-
spell idiom as fast as possible was a deliberate decision 
at the time.

----------------------------------------------------------------------

Comment By: Jeremy Hylton (jhylton)
Date: 2002-03-07 20:06

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If I understand the checkin message Guido wrote for 2.113, 
he didn't intend the current behavior.

> file_getiter(): make iter(file) be equivalent to 
>file.xreadlines().
> This should be faster.
>
> This means:
>
> (1) "for line in file:" won't work if the xreadlines 
module can't be
>    imported.
>
> (2) The body of "for line in file:" shouldn't use the 
file directly;
>     the effects (e.g. of file.readline(), file.seek() or 
even
>     file.tell()) would be undefined because of the 
buffering that goes
>     on in the xreadlines module.


----------------------------------------------------------------------

Comment By: Jason Orendorff (jorend)
Date: 2002-03-04 01:47

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Agreed on all points of fact.  Also +1 on fixing it
by making iter(f).next() and f.readline() equivalent,
one way or another.

...The easy way: make f.__iter__() call readline()
instead of being so aggressive.  (Better than nothing,
in my view.)

...The hard way (JvR's proposal): add a level of input
buffering on top of what the C runtime provides.
xreadlines() breaks user expectations precisely
because it does this *halfway*.  Doing it right would
not be such a maintenance burden, I think.  I'm willing
to write the patch, although others wiser in the ways
of diverse stdio implementations (<wink>) might want
to supervise.

...As it stands, iter(f) seems like a broken
optimization.  Which is to say: it's not "undocumented
surprising behavior"; it's a bug.


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