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Python dict KeywordUse the dict keyword to create dictionaries with other dictionaries and lists. Create an empty dictionary.
Dict. In Python a dictionary can be created with an expression in curly brackets. But the dict built-in method can also be used. This provides more initialization options.
With dict, we create an empty dictionary. We create a dictionary from another dictionary (copying it). We create one from a list.
Empty dictionary example. Here we create an empty dictionary with dict(). No keys or values are inside the dictionary. Its len is 0.
Len
Then We add a key "cat" to the dictionary with a value of 1. Then we use get() to print this value.
Python program that uses dict
# Use dict to create empty dictionary. values = dict() # Add value and get value. values["cat"] = 1 print(values.get("cat"))
1
Copy dictionary. Here we copy a dictionary with dict. We create a dictionary called values and then copy it into a dictionary called "copy."
Tip When we modify the copied dictionary, the original dictionary is left unchanged. The two are separate in memory.
Python program that copies with dict built-in
values = {"cat": 1, "bird": 2} # Copy the dictionary with the dict copy constructor. copy = dict(values) # Change copy. copy["cat"] = 400 # The two dictionaries are separate. print(values) print(copy)
{'bird': 2, 'cat': 1} {'bird': 2, 'cat': 400}
List, dict. A list of tuples (each a key-value pair) can be used to construct a dictionary. Dict turns these pairs into keys and values. This is a way to transform a list into a dictionary.
Convert
Python program that creates list with dict
# Create a dictionary with dict based on a list of pairs. # ... List contains tuples with keys and values. values = [("cat", 1), ("bird", 200)] lookup = dict(values) print(values) print(lookup.get("cat")) print(lookup.get("bird"))
[('cat', 1), ('bird', 200)] 1 200
Named arguments. Python supports named arguments. In this syntax, arguments are passed with an equals sign statement. A dictionary can be constructed with this syntax.
Def
Python program that uses named arguments
# Create a dictionary with named arguments. animals = dict(bird=1, dog=2, fish=9) print(animals.get("bird")) print(animals.get("dog")) print(animals.get("fish"))
1 2 9
Dict TypeError. The dict keyword must not be incorrectly used. If we provide incorrect arguments, a TypeError will interrupt our program's execution.
Python program that causes error
# The dict built-in must be called with a valid argument. result = dict(1)
Traceback (most recent call last): File "C:\programs\file.py", line 5, in <module> result = dict(1) TypeError: 'int' object is not iterable
Dictionary copy. We can invoke the copy() method on a dictionary to perform a copy. This has the same result as using the dict built-in method.
With this function, we create a powerful dictionary collection from varied arguments. Often with dictionaries we do not need a dict() call. But when we do, dict simplifies our code.
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