How to Retrieve Items from a Dictionary in the Order That They'Re Inserted

How do you retrieve items from a dictionary in the order that they're inserted?

The standard Python dict does this by default if you're using CPython 3.6+ (or Python 3.7+ for any other implementation of Python).

On older versions of Python you can use collections.OrderedDict.

Python: iterating through a dict in the order in which items were inserted

You shouldn't convert your dict to an OrderedDict, you should be using an OrderedDict in the first place:

from collections import OrderedDict

od = OrderedDict()
od['c'] = 1
od['f'] = 2
od['a'] = 3

Or often easier, create the OrderedDict from a list of tuples or a tuple of tuples:

od = OrderedDict([('c', 1), ('f', 2), ('a', 3)])

The problem with converting a dict to an OrderedDict is that as soon as you create a dict or add anything into it, the order is lost. So you can't use a dict at all if you need to keep the order.

Why dictionary values aren't in the inserted order?

Dictionaries in Python are unordered by definition. Use OrderedDict if you need the order in which values were inserted (it's available in Python 2.7 and 3.x).

How to access the first and the last elements in a dictionary?

Use an OrderedDict, because a normal dictionary doesn't preserve the insertion order of its elements when traversing it. Here's how:

# import the right class
from collections import OrderedDict

# create and fill the dictionary
d = OrderedDict()
d['first']  = 1
d['second'] = 2
d['third']  = 3

# retrieve key/value pairs
els = list(d.items()) # explicitly convert to a list, in case it's Python 3.x

# get first inserted element 
els[0]
=> ('first', 1)

# get last inserted element 
els[-1]
=> ('third', 3)

Order of keys in dictionaries in old versions of Python

You could use OrderedDict (requires Python 2.7) or higher.

Also, note that OrderedDict({'a': 1, 'b':2, 'c':3}) won't work since the dict you create with {...} has already forgotten the order of the elements. Instead, you want to use OrderedDict([('a', 1), ('b', 2), ('c', 3)]).

As mentioned in the documentation, for versions lower than Python 2.7, you can use this recipe.

Why items order in a dictionary changed in Python?

May I know why this is happening?

It is because of the way dicts are organized internally.

In short, this works via a hash-table which puts the keys into buckets according to their hash() value.

If I use dict.keys() to extract the keys from a dictionary and iterate it in an order that I suppose it to be, will that cause dismatch problem?

Depending on how you do it.

k = list(d.keys())
k.sort()
for i in k: print i, d[i]

should exactly work how you want it to work.

How to maintain the order of an existing dictionary python

In Python 3.6, dictionaries are ordered internally, but this is considered an implementation detail which should not be relied upon.

In Python 3.7, dictionaries are ordered.

Therefore, you have 2 options:

Use the implementation detail at your risk

You can use list(d) to retrieve the keys of the dictionary maintaining insertion order.

dirs_dictionary = {"user_dir_treatment":"/home/dataset_1/treatment",
                   "user_dir_control":"/home/dataset_1/control"}

empty_list = list(dirs_dictionary)

print(empty_list)

# ['user_dir_treatment', 'user_dir_control']

Use OrderedDict

from collections import OrderedDict

dirs_dictionary = OrderedDict([("user_dir_treatment", "/home/dataset_1/treatment"),
                               ("user_dir_control", "/home/dataset_1/control")]

empty_list = list(dirs_dictionary)

print(empty_list)

# ['user_dir_treatment', 'user_dir_control']

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