Added is_iterator method and fixes sage to use it.

[nthiery]

The following mantra occurs at three places in Sage's code to test whether v is an iterator:

     if hasattr(v, 'next'):

This is not quite correct since some sage objects have a next method without being iterable, or with a different semantic.

Let me quote python's doc:

The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:

iterator.iter()::

 Return the iterator object itself. This is required to allow both containers and iterators to be used with the for and in statements. This method corresponds to the tp_iter slot of the type structure for Python objects in the Python/C API.

iterator.next()::

Return the next item from the container. If there are no further items, raise the StopIteration exception. This method corresponds to the tp_iternext slot of the type structure for Python objects in the Python/C API.

Therefore here is the good way to test if an element is an iterator:

    try:
        return it is iter(it)
    except:
        return False

Note: it is not sufficient to check for the existence of the methods since some sage object implement __iter__ to raise a NotImplemented exception !

Florent

CC: @sagetrac-sage-combinat @williamstein

Component: misc

Keywords: iterators, itertools

Author: Florent Hivert

Reviewer: Nicolas M. Thiéry

Merged: sage-4.2.1.rc0

Issue created by migration from https://trac.sagemath.org/ticket/7398

[hivert]

Description changed:

--- 
+++ 
@@ -1,16 +1,35 @@
 The following mantra occurs at three places in Sage's code to test whether v is an iterator:
 
+```
      if hasattr(v, 'next'):
+```
+This is not quite correct since some sage objects have a next method without being iterable, or with a different semantic. 
 
-This patches replaces them with:
+Let me quote python's doc:
 
-     if hasattr(v, '__iter__')
+> The iterator objects themselves are required to support the following two methods, which together form the iterator protocol:
 
-which is safer (some sage objects have a next method without being iterable, or with a different semantic)
+ 
+>  iterator.__iter__()::
+> 
 
-if not just, when appropriate:
+     Return the iterator object itself. This is required to allow both containers and iterators to be used with the for and in statements. This method corresponds to the tp_iter slot of the type structure for Python objects in the Python/C API.
 
-     v = iter(v)
+> iterator.next()::
+> 
+
+    Return the next item from the container. If there are no further items, raise the StopIteration exception. This method corresponds to the tp_iternext slot of the type structure for Python objects in the Python/C API.
 
 
-    
+Therefore here is the good way to test if an element is an iterator:
+
+```
+    try:
+        return it is iter(it)
+    except:
+        return False
+```
+Note: it is not sufficient to check for the existence of the methods since some sage object implement `__iter__` to raise a `NotImplemented` exception !
+
+Florent
+ 

[hivert]

comment:2

Attachment: trac_7398_iter_detect-fh.patch.gz

[nthiery]

Attachment: trac_7398_iter_detect-fh.2.patch.gz

Updated patch after review by Nicolas (uses itertools to simplify further sage.server.interact.list_of_first_n)

[nthiery]

comment:4

William: this makes a small edit to sage.server.notebook.interact.list_of_first_n

You may want to check/backport to the notebook code

[nthiery]

Description changed:

--- 
+++ 
@@ -32,4 +32,4 @@
 Note: it is not sufficient to check for the existence of the methods since some sage object implement `__iter__` to raise a `NotImplemented` exception !
 
 Florent
- 
+

[nthiery]

Changed keywords from iterators to iterators, itertools

[hivert]

comment:5

The given patch actually breaks somme code... I'm uploading a new one.