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Lourenço Costa
Lourenço Costa

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Python course: Deconstructing

#python #books #tutorial

This is a process of extracting values from data structures such as lists, tuples, and dicts, and assigning them to isolated variables in a single statement. This is also known as "unpacking".

name, position = ("oscar", "accountant") 
print(name) # => oscar
print(position) # => accountant

# In dicts we use the values (method ".values()") below
branch_name, active = {"name": "scranton", "is_active": True}.values() 

print(branch_name) # => scranton
print(active) # => True
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A few remarks:

  • Notice that the order matters here. The variables will match their respective position in the collection.
  • When deconstructing a dict, the variable's names do not need to match the keys' names. Only the position matters. See that I deconstructed branch_name to the "name" key and active to the "is_active" key.

Another interesting use case is to use deconstructed values as function arguments.

def print_names(*names): 
    for i in names:
        print(i)

employee_list = ["oscar", "meredith", "pam"]
print_names("oscar", "meredith", "pam") 
print_names(*employee_list)
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The output of both calls to print_names() is the same:

oscar 
meredith 
pam
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And another example using a dict as a **kwargs argument:

def print_employee_info(name: str, age: int, gender: str): 
    print("Name:", name)
    print("Age:", age) 
    print("Gender:", gender)

employee = {"gender": "male", "age": 37, "name": "Dwight"} 
print_employee_info(**employee)
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Output:

Name: Dwight 
Age: 37 
Gender: male
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Notice that in the previous example, the order of the keys in the employee dict doesn't match the respective deconstructed variables described as parameters in the print_employee_info() function. So, based on the rules I mentioned earlier, it should not work. But it does work!

The reason it works is because employee was passed to the function as a ** kwargs argument. If you recall from the Functions post, there's a very distinct difference in calling a function with unnamed arguments (* args) and named arguments (** kwargs).

As a result, what matters here is that the keys in employee have an exact match to the parameters' names in the print_employee_info() function.

In other words, employee["name"] gets mapped to name, employee["gender"] to gender, and employee["age"] to age, regardless of their position in the dict!

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