Python on DonDoIT

Python_async

Thu, 13 Feb 2025 21:42:52 +0200

1. Is it still single threaded when using python async/await?

Yes!

When an async function e.g named task1 runs and hits the await keyword, the event loop pauses the execution of task1 and delegates the work to:
- OS Kernel (for I/O)
- System timers (for sleep)
- Thread pool (for CPU-bound work, if used explicitly)
After that, the event loop continues and pick up another async function in its queue to execute.
While the event loop is executing other tasks, if the OS finishes working on the delegated job and gives back the result. The result is then put into a “result queue”.
When all the async tasks in the event loop’s queue are executed, the event loop will pick up the results from the results queue, in the order of first come first serve.
The event loop will now continue the execution of the task of first result that gets pickup, and continue until it hits another await or finished the execution.
Async functions (async def) run in the same thread and do not block the event loop as long as they contain await.
Regular (def) functions block the event loop unless explicitly run in a separate thread or process.

2. What kind of tasks would be best to use `async/await`?

Things like I/O, sleep, network requests, file reads.
Best practice: Keep async functions fully async and offload blocking operations to threads or processes when necessary.

3. Prevent blocking?

To prevent blocking, use:

asyncio.to_thread() → Runs a regular function in a separate thread (good for I/O).
asyncio.run_in_executor() with ProcessPoolExecutor → Runs CPU-heavy tasks in a separate process (avoids GIL limitations).

Regex in Python (part 3)

Thu, 28 Nov 2024 19:16:14 +0200

Find expression containing numbers and symbols in a specific format

Assuming that we have this piece of text that contains an IPv4 address that we want to extract.

import re

text = "You've recently logged in from an IP address 111.222.211.122"

The full range of IP addresses can go from 0.0.0.0 to 255.255.255.255, so we can use the following regex pattern to search

pattern = "\d\d\d.\d\d\d.\d\d\d.\d\d\d"

The result of this will be

>>> print(re.findall(pattern, text))
>>> ['111.222.211.122']

However, if the text now has something extra like this:

text = "You've recently logged in from an IP address 111.222.211.122, and something weird like this 123123123123122"

now our search result will be ['111.222.211.122', '123123123123122']. This is because:

\d\d\d will try to match any 3 digit numbers
. actually will try to match anything, so 1231 would match with the pattern \d\d\d.. The same goes for 123! or 123@.

If we want to specifically match the dot ., add a backslash \ in front of the dot. It’s going to be like this: \d\d\d\.

Extra tips:

the pattern \d+ will help matching a number of any length

Regex in Python (part 2)

Thu, 21 Nov 2024 18:47:19 +0200

Find words of specifc length starting with specific letter

Assuming we want to search for all the 2-character words that start with an i and ends with e.g s, t, o, n, l

import re


text = "I live in Finland and the cold is killing me"
pattern = "i[stonl]"

matches = re.findall(pattern, text)

When printing the result matches, you’ll get ['in', 'in', 'is', 'il', 'in'], which come from

I live [in] F[in]land and the cold [is] k[il]l[in]g me

This pattern is not only finding a word, but also sub-string of a word that matches the pattern.

With `^`

When we modify our pattern a bit by adding ^ so that it would look like this

pattern = "^i[stonl]"

Now our matches result will be []. This is because ^ indicate that we’re looking for the word or substring of a word, which is at the beginning of the text, or in other word, in this case, i must be the first character in our text.

How about "^i[stonl][nm]"? This means that we’re searching for a substring of a word, which starts with i as the first letter in the text, follow by one of the characters s, t, o, n, l and ends with either n or m.

Extra tips:

We can use $ to search for the pattern at the end of a line. For example, r"me$" will find any word that ends with me at the end of the line.

If we want to specifically search for an independent word, use \b. For example:

>>> text = "This is example0 and example1"
>>> pattern = r"\bexample[01]?\b"
>>> print(re.findall(pattern, text))
>>> ["example0", "example1"]

Hope this is helpful 😊.

Regex in Python (part 1)

Thu, 21 Nov 2024 00:05:47 +0200

Search for string in text

Assuming that we have a text the quick brown fox jumped over the lazy dog, and we want to search for e.g quick in the text.

import re

text = "the quick brown fox jumped over the lazy dog"

match = re.search("quick", text)

As said in .search() documentation, this method will look for the first location where it finds a match, and returns a re.Match object if found, otherwise returns None.

If we print(match), we’ll see <re.Match object; span=(4, 9), match='quick'> which indicate that the matching string starts at the index 4 and ends at index 9 exclusively.

To get the matched value that the re.Match object is holding, we can simply use

match.group()

Find characters by type

Assuming we’re now working with a slightly different bit of text from the example above

import re

text = "the quick brown fox jumped over the lazy dog 1234567890 !@#$%^&*()_"

Find alphanumeric characters

To find all the word characters, we can use regex expression \w.

characters = re.findall("\w", text)

When printing the result characters, we’ll get all the characters in the text splited into a list, however, !@#$%^&*() won’t be returned as they are not considered word characters, except _.

['t','h','e','q','u','i','c','k','b','r','o','w','n','f','o','x','j','u','m','p','e','d','o','v','e','r','t','h','e','l','a','z','y','d','o','g','1','2','3','4','5','6','7','8','9','0','_']

Find any characters

To find any character, doesn’t matter if it’s word character or not, use .

any_characters = re.findall(".", text)

Note that now the result also contains whitespaces ' '

['t','h','e',' ','q','u','i','c','k',' ','b','r','o','w','n',' ','f','o','x',' ','j','u','m','p','e','d',' ','o','v','e','r',' ','t','h','e',' ','l','a','z','y',' ','d','o','g',' ','1','2','3','4','5','6','7','8','9','0',' ','!','@','#','$','%','^','&','*','(',')','_']

Find non-word characters

Opposite to \w, we have \W (uppercase) that we can use to find all non-word characters

non_word_characters = re.findall(".", text)

The result now only contains whitespaces and symbols characters

[' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', ' ', '!', '@', '#', '$', '%', '^', '&', '*', '(', ')']