Modifying a Python dict while iterating over it

Question

Let's say we have a Python dictionary d, and we're iterating over it like so:

for k,v in d.iteritems():     del d[f(k)] # remove some item     d[g(k)] = v # add a new item 

(f and g are just some black-box transformations.)

In other words, we try to add/remove items to d while iterating over it using iteritems.

Is this well defined? Could you provide some references to support your answer?

(It's pretty obvious how to fix this if it's broken, so this isn't the angle I am after.)

Answer 1

Alex Martelli weighs in on this here.

It may not be safe to change the container (e.g. dict) while looping over the container. So del d[f(k)] may not be safe. As you know, the workaround is to use d.items() (to loop over an independent copy of the container) instead of d.iteritems() (which uses the same underlying container).

It is okay to modify the value at an existing index of the dict, but inserting values at new indices (e.g. d[g(k)]=v) may not work.

Answer 2

It is explicitly mentioned on the Python doc page (for Python 2.7) that

Using iteritems() while adding or deleting entries in the dictionary may raise a RuntimeError or fail to iterate over all entries.

Similarly for Python 3.

The same holds for iter(d), d.iterkeys() and d.itervalues(), and I'll go as far as saying that it does for for k, v in d.items(): (I can't remember exactly what for does, but I would not be surprised if the implementation called iter(d)).