Python 3.8 New Features, The Walrus Operator And Positional-Only Parameters
Section Overview
- The Walrus Operator
- Positional-Only Parameters
- References
Section 1. The Walrus Operator (Assignment Expressions)
We use an assignment expression(walrus operator). It use the walrus operator (:=) to both assign and evaluate variable names in a single expression, thus reducing repetition. It is a new syntax introduced in Python 3.8 and solve a long-standing problem with the language that can cause code duplication.
Now, let’s see:
if fake_val := fake_func():
# do something with fake_val
while fake_val := fake_func():
# do something with fake_val
fake_val = fake_func1()
if not fake_val:
logging.warning('fake func1')
fake_val = fake_func2()
if not fake_val:
logging.warning('fake func2')
val = func3()
if not fake_val:
raise ValueError('....')
After upgrading to 3.8, you could refactor it to:
if not (fake_val := fake_func1()):
logging.warning('fake func1')
if not (fake_val := fake_func2()):
logging.warning('fake func2')
if not (fake_val := fake_func3()):
raise ValueError('....')
For example, Here, I define the contents of the basket.
fruits_in_basket = {
'apple': 15,
'peach': 10,
'lemon': 5,
}
Now we want to order some lemon. So we need to make sure there is at least one lemon in the basket.
def order_func(count):
...
def out_of_stock():
...
count = fruits_in_basket.get('lemon', 0)
if count:
order_func(count)
else:
out_of_stock()
After upgrading to 3.8, you could refactor it to:
if count := fruits_in_basket.get('lemon', 0):
order_func(count)
else:
out_of_stock()
If we want to do lemon juice, we need 10 lemon.
def make_lemon_juice(count):
...
count = fruits_in_basket.get('lemon', 0)
if count >= 10:
make_lemon_juice(count)
else:
out_of_stock()
After upgrading to 3.8, you could refactor it to:
if (count := fruits_in_basket.get('lemon', 0)) >= 10:
make_lemon_juice(count)
else:
out_of_stock()
If we want to order for peach and lemon juice;
def make_peach_juice(count):
...
def make_lemon_juice(count):
...
count = fruits_in_basket.get('peach', 0)
if count >= 2:
make_peach_juice(count)
else:
count = fruits_in_basket.get('lemon', 0)
if count >= 10:
make_lemon_juice(count)
else:
# 'Nothing'
After upgrading to 3.8, you could refactor it to:
if (count := fruits_in_basket.get('peach', 0)) >= 2:
make_peach_juice(count)
elif (count := fruits_in_basket.get('lemon', 0)) >= 10:
make_lemon_juice(count)
else:
# 'Nothing'
In general, when you find yourself repeating the same expression or assignment multiple times within a grouping of lines, it’s time to consider using assignment expressions in order to improve readability.
Section 2. Positional-Only Parameters
We can use / in a function definition to indicate that arguments before that are positional-only.
For example:
def func(a, /):
pass
func('foobar')
# This raises
func(a='foobar')
If the func uses keyword argument when positional-only is required, TypeError occurs.
Here’s an example function definition which includes all argument types.
def func(a, /, b, *, c):
pass
a: positional-only
b: positional or keyword
c: keyword-only
Thus, func can be called like this:
func('foo', 'bar', c='foobar')
func('foo', b='bar', c='foobar')
But not like this:
# TypeError: func() got some positional-only arguments passed as keyword arguments: 'a'
func(a='foo', b='bar', c='foobar')
# TypeError: func() takes 2 positional arguments but 3 were given
func('foo', 'bar', 'foobar')
Section 3. References
- https://www.python.org/dev/peps/pep-0569/#id6
- https://docs.python.org/3.8/whatsnew/3.8.html
- https://www.python.org/dev/peps/pep-0572/
- https://www.python.org/dev/peps/pep-0013/#electing-the-council
- https://docs.python.org/3/glossary.html#keyword-only-parameter