Commit 68b76381 authored by Tim Peters's avatar Tim Peters

Added long-requested docs about mutating BTrees while iterating over them. ...

Added long-requested docs about mutating BTrees while iterating over them.  Will regen the .pdf and .html later (I want to add more .tex docs first).
parent 2bae71ec
...@@ -3,6 +3,7 @@ ...@@ -3,6 +3,7 @@
% PersistentList % PersistentList
% BTrees % BTrees
% Total Ordering and Persistence % Total Ordering and Persistence
% Iteration and Mutation
% BTree Diagnostic Tools % BTree Diagnostic Tools
\section{Related Modules} \section{Related Modules}
...@@ -362,6 +363,51 @@ only with great care: ...@@ -362,6 +363,51 @@ only with great care:
\end{enumerate} \end{enumerate}
\subsubsection{Iteration and Mutation}
As with a Python dictionary or list, you should not mutate a BTree-based
data structure while iterating over it, except that it's fine to replace
the value associated with an existing key while iterating. You won't
create internal damage in the structure if you try to remove, or add new
keys, while iterating, but the results are undefined and unpredictable. A
weak attempt is made to raise \exception{RuntimeError} if the size of a
BTree-based structure changes while iterating, but it doesn't catch most
such cases, and is also unreliable. Example:
\begin{verbatim}
>>> from BTrees.IIBTree import *
>>> s = IISet(range(10))
>>> list(s)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> for i in s: # the output is undefined
... print i,
... s.remove(i)
0 2 4 6 8
Traceback (most recent call last):
File "<stdin>", line 1, in ?
RuntimeError: the bucket being iterated changed size
>>> list(s) # this output is also undefined
[1, 3, 5, 7, 9]
>>>
\end{verbatim}
Also as with Python dictionaries and lists, the safe and predictable way
to mutate a BTree-based structure while iterating over it is to iterate
over a copy of the keys. Example:
\begin{verbatim}
>>> from BTrees.IIBTree import *
>>> s = IISet(range(10))
>>> for i in list(s.keys()): # this is well defined
... print i,
... s.remove(i)
0 1 2 3 4 5 6 7 8 9
>>> list(s)
[]
>>>
\end{verbatim}
\subsubsection{BTree Diagnostic Tools} \subsubsection{BTree Diagnostic Tools}
A BTree (or TreeSet) is a complex data structure, really a graph of A BTree (or TreeSet) is a complex data structure, really a graph of
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