Olivier Bénard

The Python positional-only parameter has been introduced with the Python 3.8 release. It is a way to specify which parameters must be positional-only arguments – i.e. which parameters cannot be given as keyword-parameters. It uses the / parameter syntax. Elements positioned on the left side will be then turned into positional-only parameters.

def f(a, b, /, c): ...

The above method will only accepts calls of the following form:

python> f(1, 2, 3)
python> f(1, 2, c=3)

Note: c can be either given a position or keyword argument.

On the contrary, the following calls will raise a TypeError: f() got some positional-only arguments passed as keyword arguments error as a and b – being on the left of / – cannot be exposed as possible keyword-arguments:

python> f(1, b=2, 3)
python> f(1, b=2, c=3)
python> f(a=1, b=2, 3)

Use cases

The / positional-only parameter syntax is helpful in the following use-cases:

• It precludes keyword arguments from being used when the parameter’s name is not necessary, confusing or misleading e.g. len(obj=my_list). As stated in the documentation, the obj keyword argument would here impairs readability.

• It allows the name of the parameter to be changed in the future without introducing breaking changes in he client as they can still be passed through **kwargs to be used in two ways:

def transform(name, /, **kwargs):
    print(name.upper())

In the above snippet, you can access the name‘s value either using name or kwargs.get("name").

Last but not the least, it can also be used to prevent overwriting pre-set keyword-arguments in methods that still need to accept arbitrary keyword arguments (via kwargs):

def initialize(name="unknown", /, **kwargs):
    print(name, kwargs)

The name keyword-argument is protected and could not be overwritten, even if mistakenly captured a second time by the kwargs argument:

python> initialize(name="olivier")
unknown {'name': 'olivier'}

Notes:

• Python does not allow positional arguments after keyword arguments because of the left-side/right-side of the / operator thingy.
• Python does not allow positional arguments after keyword arguments because of the left-side/right-side of the * operator thingy.
• *args are collecting as positional arguments.
• **kwargs are collecting as keyword-arguments.

pickle allows you to serialize and de-serialize Python objects to save them into a file for future use. You can then read this file and extract the stored Python objects, de-serializing them so they can be integrated back into the code’s logic.

You just need the two basic commands: pickle.dump(object, file) and pickle.load(file).

Below, a round trip example:

import pickle

FILENAME = "tmp.pickle"
original_list = ["banana", "apple", "pear"]

with open(FILENAME, "wb") as file:
    pickle.dump(original_list, file)
file.close()

with open(FILENAME, "rb") as file:
    retrieved_list = pickle.load(file)
file.close()

print(retrieved_list) # ["banana", "apple", "pear"]

The itertools.cycle() method is a nice way to iterate through an iterable – e.g. a list – indefinitely in a cycling way. When all the elements are exhausted, the elements are once again red from the beginning.

from itertools import cycle

duty_persons = ["Olivier", "David", "Goran",]
duty_persons_cycle = cycle(duty_persons)

for _ in range(6):
    print(next(duty_persons_cycle))

The above snippet will returned the following output:

Olivier
David
Goran
Olivier
David
Goran

As you can see in the above example, the cycle method is quite helpful in a couple of situations:

• establishing a rolling list of duty-persons, rotating on a regular base;
• can be used when scrapping the web to cycle between hosts to outrun anti-bot policies;
• any use-case you might think of…

uuid is a python module providing immutable UUID (Universally Unique IDentifier URN Namespace) objects and functions e.g. uuid4().

The generated UUID objects are unique since it generates IDs based on time and computer hardware.

python> from uuid import uuid4
python> uuid_object = uuid4()
python> uuid_object
UUID('ce8d1fee-bc31-406b-8690-94c01caabcb6')

python> str(uuid_object)
'ce8d1fee-bc31-406b-8690-94c01caabcb6'

Those can be used to generate random strings that can serve as unique identifier across a given namespace – e.g. if you want to generate temporary filenames on the fly located under a specific folder:

for _ in range(10):
    file = open(f"/tmp/{uuid4()}.txt", "a")
    file.write("hello world!")
    file.close()

Note: despite the above snippet working like a charm, it is better to use open() with the context manager with to make sure the close() function will always be called should an error occurs during the write operation.

Hazardous: uuid1() compromises privacy since it uses the network address of the computer to generate the unique ID. Thus, it could be reverse-engineered and retrieved. To prevent this, always choose one of the latest functions e.g. uuid4() or uuid5() as the previous ones rapidly get depreciated.

Note: to know more about URIs, URLs and URNs https://olivierbenard.fr/change-the-url-of-a-git-repository/.

• append() appends the given object at the end of the list. The length is only incremented of +1.
• extend() extends the list by appending the elements it contains one by one into the list. The length is incremented by the number of elements.

In more details

The append() method is straight forward:

python> a = [1,2,3]
python> a.append(4)
python> a
[1, 2, 3, 4]

However, what happens if you need to add multiple elements at the same time? You could call the append() method several times, but would it work to give the list of new elements to be added into the original list directly as a parameter? Let’s give a try:

python> a = [1,2,3]
python> a.append([4,5])
python> a
[1, 2, 3, [5, 6]] # vs. expected [1, 2, 3, 4, 5, 6]

The extend() method intends to solve this problem:

python> b = [1,2,3]
python> b.append([4,5])
python> b
[1, 2, 3, 4, 5]

Note: the + operator is equivalent to an extend call.

python> a = [1,2,3]
python> a += [4,5]
python> a
[1, 2, 3, 4, 5]