Python

What's new in Python 3.8 and 3.9¶

Author: Marcel Baltruschat (@GitHub)
Date: 04.02.2021

Content¶

Dictionary Merge Operator
Assignment Expressions
New String Methods to Remove Prefixes and Suffixes
f-strings support = for self-documenting expressions and debugging
Positional-only parameters

Dictionary Merge Operator¶

https://docs.python.org/3/whatsnew/3.9.html#dictionary-merge-update-operators

In [1]:

d1 = {'x': 1, 'y': 2}
d2 = {'y': 3, 'z': 0}

In [2]:

# Before
{**d1, **d2}

Out[2]:

{'x': 1, 'y': 3, 'z': 0}

In [3]:

# Now
d1 | d2

Out[3]:

{'x': 1, 'y': 3, 'z': 0}

In [4]:

# Important: The last item "wins"

Assignment Expressions¶

https://docs.python.org/3.8/whatsnew/3.8.html#assignment-expressions

In [5]:

a = [1, 2, 3, 4, 5, 6]

In [6]:

# Before
n = len(a)
if n > 5:
    print(f'List with {n} elements is too long!')

List with 6 elements is too long!

In [7]:

# Now
if (n := len(a)) > 5:
    print(f'List with {n} elements is too long!')

List with 6 elements is too long!

New String Methods to Remove Prefixes and Suffixes¶

https://docs.python.org/3/whatsnew/3.9.html#new-string-methods-to-remove-prefixes-and-suffixes

In [8]:

import os

In [9]:

s1 = 'myfile.txt'

In [10]:

# Before
s1.split('.')[0]                  # Problem with multiple dots in filename
s1[:-4]                           # Problem if you don't know the length
''.join(s1.split('.')[:-1])       # Complicated
os.path.splitext('myfile.txt')[0] # Needs import

Out[10]:

'myfile'

In [11]:

# Now
s1.removesuffix('.txt')  # You need to know the fileending

Out[11]:

'myfile'

In [12]:

s2 = 'www.czodrowskilab.org'
s2.removeprefix('www.')

Out[12]:

'czodrowskilab.org'

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

https://docs.python.org/3.8/whatsnew/3.8.html#f-strings-support-for-self-documenting-expressions-and-debugging

In [13]:

from datetime import date
from math import cos, pi

In [14]:

user = 'John Doe'

In [15]:

# Before
f'user={user}'

Out[15]:

'user=John Doe'

In [16]:

# Now
f'{user=}'

Out[16]:

"user='John Doe'"

In [17]:

f'{user=!s}'

Out[17]:

'user=John Doe'

In [18]:

# Before
a = cos(2**6 + pi)
f'{a:.3f}'

Out[18]:

'-0.392'

In [19]:

# Now
f'{cos(2**6 + pi)=:.3f}'

Out[19]:

'cos(2**6 + pi)=-0.392'

Positional-only parameters¶

https://docs.python.org/3.8/whatsnew/3.8.html#positional-only-parameters

In [20]:

def f(a, /, b, *, c):
    # a has to be positional
    # b can be positional or a keyword argument
    # c has to be a keyword argument
    print(a, b, c)

In [21]:

# Valid calls
f(1, 2, c=3)
f(1, b=2, c=3)

1 2 3
1 2 3

In [22]:

# Invalid calls
f(1, 2, 3)
f(a=1, b=2, c=3)

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-22-a83ad21307fb> in <module>
      1 # Invalid calls
----> 2 f(1, 2, 3)
      3 f(a=1, b=2, c=3)

TypeError: f() takes 2 positional arguments but 3 were given