Is the order of a Python dictionary guaranteed over iterations?

Solution 1:

Yes, the same order is guaranteed if it is not modified.

See the docs here.

Edit:

Regarding if changing the value (but not adding/removing a key) will affect the order, this is what the comments in the C-source says:

/* CAUTION: PyDict_SetItem() must guarantee that it won't resize the
 * dictionary if it's merely replacing the value for an existing key.
 * This means that it's safe to loop over a dictionary with PyDict_Next()
 * and occasionally replace a value -- but you can't insert new keys or
 * remove them.
 */

It seems that its not an implementation detail, but a requirement of the language.

Solution 2:

It depends on the Python version.

Python 3.7+

Dictionary iteration order is guaranteed to be in order of insertion.

Python 3.6

Dictionary iteration order happens to be in order of insertion in CPython implementation, but it is not a documented guarantee of the language.

Prior versions

Keys and values are iterated over in an arbitrary order which is non-random, varies across Python implementations, and depends on the dictionary’s history of insertions and deletions. If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond. https://docs.python.org/2/library/stdtypes.html#dictionary-view-objects

The -R option

Python 2.6 added the -R option as (insufficient, it turned out) protection against hash flooding attacks. In Python 2 turning this on affected dictionary iteration order (the properties specified above were still maintained, but the specific iteration order would be different from one execution of the program to the next). For this reason, the option was off by default.

In Python 3, the -R option is on by default since Python 3.3, which adds nondeterminism to dict iteration order, as every time Python interpreter is run, the seed value for hash computation is generated randomly. This situation lasts until CPython 3.6 which changed dict implementation in a way so that the hash values of entries do not influence iteration order.

Source

Changed in version 3.7: Dictionary order is guaranteed to be insertion order. This behavior was an implementation detail of CPython from 3.6. https://docs.python.org/3.8/library/stdtypes.html

What’s New In Python 3.6: The order-preserving aspect of this new implementation is considered an implementation detail and should not be relied upon (this may change in the future, but it is desired to have this new dict implementation in the language for a few releases before changing the language spec to mandate order-preserving semantics for all current and future Python implementations; this also helps preserve backwards-compatibility with older versions of the language where random iteration order is still in effect, e.g. Python 3.5). https://docs.python.org/3/whatsnew/3.6.html#whatsnew36-compactdict