How to create key or append an element to key?

Advanced manipulations with dictionaries

Take note of different ways you can play around with adding and appending elements to a dictionary, depending on your use case and Python version.

Option 1: Using collections.defaultdict

The collections.defaultdict is a handy tool that makes interacting with dictionaries more intuitive by automatically creating missing keys:

from collections import defaultdict

my_defaultdict = defaultdict(list)
my_defaultdict['my_key'].append('my_value') # The key is created if it doesn't exist.
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Option 2: Using try/except blocks

You can apply the EAFP (Easier to Ask for Forgiveness than Permission) principle using a try/except block to manage key existence:

try:
    my_dict['my_key'].append('my_value')  # Let's see if the key exists.
except KeyError:
    my_dict['my_key'] = ['my_value']  # Oops! It doesn't. Let's create it.
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Option 3: The .get() method

Here we can retrieve a key with a default value and then update the dictionary if necessary:

my_list = my_dict.get('my_key', [])
my_list.append('my_value')
my_dict['my_key'] = my_list  # Our dictionary has now grown up a bit.
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Picking the best method for your dictionaries

The selection between setdefault, defaultdict, and get should balance both performance and readability needs of your application.

Performance considerations

While setdefault may create redundant lists, it's only a bottleneck for applications working with large datasets. If efficiency is critical, defaultdict might be your best friend.

Readability and code neatness

When it comes to readability, defaultdict often takes the trophy as it reduces unnecessary key existence checks, leading to cleaner code.

Python version compatibility

Remember, your Python version also matters, defaultdict may not be available in Python versions before 2.5.

To remember when dealing with dictionaries

There are various caveats and tips, specifically for Python dictionaries that are important to consider:

1. Watch out for shared references

When you use setdefault with mutable objects like lists, it could lead to shared references between keys causing unexpected behaviour.

2. Be aware of the mutation scenario

Appending to a dictionary value with .append() mutates the corresponding list. Make sure such mutation is intended in your application logic.

3. Use the appropriate dictionary key existence pattern

Both setdefault and defaultdict offer patterns to handle non-existing keys. It's important to pick which suits your code style and performance needs the best.