CODEWITHSUNDEEP
pythonconditional-statements
piquesel
piquesel

Reputation: 159

Conditional expression returning type

I don't quite understand how this piece of code works:

def sequence_class(immutable):
    return tuple if immutable else list

seq = sequence_class(immutable=False)
s = seq("Nairobi")
s
['N', 'a', 'i', 'r', 'o', 'b', 'i']
seq = sequence_class(immutable=True)
s = seq("Nairobi")
s
('N', 'a', 'i', 'r', 'o', 'b', 'i')

It is quite obvious what it is doing, but I don't understand how the function can magically return the tuple("Nairobi") or list("Nairobi") just with the statement return tuple if mutable else list and without any parameter to the function.

Any clear explanation to this?

Upvotes: 2

Views: 84

Answers (2)

Martijn Pieters
Martijn Pieters

Reputation: 1124538

Everything in Python is an object; the tuple and list objects are no exception.

The function simply returns a reference to either the tuple or the list object, based on the immutable flag:

>>> def sequence_class(immutable):
...     return tuple if immutable else list
... 
>>> sequence_class(True)
<class 'tuple'>
>>> sequence_class(False)
<class 'list'>

You then bind seq to that object, and calling seq then calls the referenced type:

>>> seq = sequence_class(True)
>>> seq
<class 'tuple'>
>>> seq('abc')
('a', 'b', 'c')

You can do the same by directly assigning tuple or list to a variable, it just creates another reference to the same object:

>>> foo = list
>>> foo
<class 'list'>
>>> foo('abc')
['a', 'b', 'c']

Upvotes: 5

Jivan
Jivan

Reputation: 23098

Calling sequence_class is exactly as if you did:

# if immutable == True
seq = tuple('Nairobi')

# if immutable == False
seq = list('Nairobi')

You just happened to replace tuple or list with the sequence_class() function, which returns tuple or list based on the argument you provide.

Each time you call sequence_class(), this expression is replaced with the value it returns.

Upvotes: 0

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