Assignment expressions

New syntax implemented (:=) that allow to do variable assignments right in the expressions.

discount = 0.0
if (mo := re.search(r'(\d+)% discount', advertisement)):
    discount = float(mo.group(1)) / 100.0

Earlier I would need to do something like that:

discount = 0.0
if re.search(r'(\d+)% discount', advertisement):
    mo = re.search(r'(\d+)% discount', advertisement)
    discount = float(mo.group(1)) / 100.0

It recommended though to limit use of so-called "walrus" operator only in clean cases to reduce complexity and improve readability.

More info can be found in PEP 572.

f-strings support "=" for self-documenting expressions and debugging

Added an = specifier to f-strings. An f-string such as f'{expr=} will expand to the text of the expression, an equal sign, then the representation of the evaluated expression.

>>> user = 'eric_idle'
>>> member_since = date(1975, 7, 31)
>>> f'{user=} {member_since=}'
"user='eric_idle' member_since=datetime.date(1975, 7, 31)"

The = specifier will display the whole expression so that calculations can be shown:

>>> print(f'{theta=}  {cos(radians(theta))=:.3f}')
theta=30  cos(radians(theta))=0.866

More details in contribution bpo-36817.

Other changes

  • A continue statement now legal in the finally clause. Earlier it was illegal due to a problem with implementation.
  • When the Python interpreter is interrupted by Ctrl-C (SIGINT) and the resulting KeyboardInterrupt exception is not caught, the Python process now exits via a SIGINT signal or with the correct exit code such that the calling process can detect that it died due to a Ctrl-C. Shells on POSIX and Windows use this to properly terminate scripts in interactive sessions.

It's required to say, that for today (Aug 14) Python 3.8 is in beta-phase and release date is not known.