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This short article teaches you 3 ways of creating a Python dictionary.

Here are 3 ways in which you can create a Python 🐍 dictionary: (Of the three, the last one is the least commonly used.)

>>> dict([(1, "one"), (2, "two")])
{1: 'one', 2: 'two'}

>>> dict(name="Rodrigo", twitter="mathsppblog")
{'name': 'Rodrigo', 'twitter': 'mathsppblog'}

>>> dict.fromkeys(["likes", "retweets"], 0)
{'likes': 0, 'retweets': 0}

An iterable of key, value pairs

The built-in dict can take an iterable with key, value pairs. Useful, for example, when you have a bunch of keys and a bunch of values that you put together with zip:

>>> dict([(1, "one"), (2, "two")])
{1: 'one', 2: 'two'}

>>> keys = range(1, 4)
>>> values = "one two three".split()
>>> dict(zip(keys, values))
{1: 'one', 2: 'two', 3: 'three'}

In this particular case, you could also get creating with enumerate and make use of the keyword argument start:

>>> dict(enumerate(values, start=1))
{1: 'one', 2: 'two', 3: 'three'}

Keyword arguments

You can use keyword arguments in dict to define key, value pairs in your dictionary! However, this only works if your keys are valid variable names:

# Values don't have to be strings:
>>> dict(one=1, two=2)
{'one': 1, 'two': 2}

# But the keys have to:
>>> dict(1="one", 2="two")
  File "<stdin>", line 1
    dict(1="one", 2="two")
         ^
SyntaxError: expression cannot contain assignment, perhaps you meant "=="?

# Keys have to be valid variable names:
>>> dict(name="Rodrigo", twitter="mathsppblog")
{'name': 'Rodrigo', 'twitter': 'mathsppblog'}

Class method dict.fromkeys

The class method dict.fromkeys accepts an iterable and a value, and produces a dictionary where all keys have that value. By default, that value is None:

# Default value is None:
>>> dict.fromkeys("abc")
{'a': None, 'b': None, 'c': None}
>>> dict.fromkeys(range(5))
{0: None, 1: None, 2: None, 3: None, 4: None}

# Different default values:
>>> dict.fromkeys(range(5), ";)")
{0: ';)', 1: ';)', 2: ';)', 3: ';)', 4: ';)'}
>>> dict.fromkeys(["likes", "retweets"], 0)
{'likes': 0, 'retweets': 0}

The class method .fromkeys has a gotcha associated with it, though. Be careful when using mutable values, because the value isn't copied to each key. It's exactly the same object used over and over:

>>> d = dict.fromkeys(range(3), [])
>>> d
{0: [], 1: [], 2: []}

>>> d[0].append("zero")
>>> d[1].append("one")

>>> d[2]  # Shouldn't d[2] be empty?!
['zero', 'one']
>>> d
{0: ['zero', 'one'], 1: ['zero', 'one'], 2: ['zero', 'one']}  # 🀯

These are just 3 ways of creating a Python dictionary. Soon, I'll send out a Mathspp Insider article talking about all the ways in which you can create dictionaries in Python 🐍 Join to keep learning: https://insider.mathspp.com

Summary

Here's a quick recap:

  1. dict accepts an iterable that contains key, value pairs;
  2. use keyword arguments in dict if you want string keys; and
  3. .fromkeys gives the same value (default is None) to a bunch of keys.

Comment other ways in which you can create a Python 🐍 dict!

This article was generated automatically from this thread I published on Twitter @mathsppblog. Then it was edited lightly.

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