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Example code for Fluent Python, 2nd edition (O'Reilly 2022)

Home Page: https://amzn.to/3J48u2J

License: MIT License

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python python3 concurrency iterators metaprogramming special-methods

example-code-2e's Introduction

Fluent Python 2e example code

Example code for the book Fluent Python, Second Edition by Luciano Ramalho (O'Reilly, 2022).

All chapters are undergoing review and updates, including significant rewrites in the chapters about concurrency in Part V.

New chapters in Fluent Python 2e are marked with 🆕.

🚨 This table of contents is subject to change at any time until the book goes to the printer.
Latest change: Old Part I—Prologue merged into new Part I—Data Structures; parts renumbered accordingly; chapter numbers unchanged.

Part / Chapter #	Title	Directory	1^st ed. Chapter #
I – Data Structures
1	The Python Data Model	01-data-model	1
2	An Array of Sequences	02-array-seq	2
3	Dictionaries and Sets	03-dict-set	3
4	Unicode Text versus Bytes	04-text-byte	4
5	Data Class Builders	05-data-classes	🆕
6	Object References, Mutability, and Recycling	06-obj-ref	8
II – Functions as Objects
7	Funcions as First-Class Objects	07-1class-func	5
8	Type Hints in Functions	08-def-type-hints	🆕
9	Decorators and Closures	09-closure-deco	7
10	Design Patterns with First-Class Functions	10-dp-1class-func	6
III – Object-Oriented Idioms
11	A Pythonic Object	11-pythonic-obj	9
12	Special Methods for Sequences	12-seq-hacking	10
13	Interfaces, Protocols, and ABCs	13-protocl-abc	11
14	Inheritance: For Better or For Worse	14-inheritance	12
15	More About Type Hints	15-more-types	🆕
16	Operator Overloading	16-op-overloading	13
IV – Control Flow
17	Iterators, Generators, and Classic Coroutines	17-it-generator	14
18	with, match, and else Blocks	18-with-match	15
19	Concurrency Models in Python	19-concurrency	🆕
20	Concurrent Executors	20-executors	17
21	Asynchronous Programming	21-async	18
V – Metaprogramming
22	Dynamic Attributes and Properties	22-dyn-attr-prop	19
23	Attribute Descriptors	23-descriptor	20
24	Class Metaprogramming	24-class-metaprog	21

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example-code-2e's Issues

Implementation details in CH21

I deployed the local server according to SETTING UP TEST SERVERS, finding two small suspected errors both in slow_server.py.

The first one is about the parser in the main fucncion(Line 59). The sokcet module requires the parameter bind to be str, bytes, or bytearray , so I add a default value as the other part. (This problem is maybe linked to the python environment thing)

The second one is in the doGet function of SlowHTTPRequestHandler(Line41). I get an AttributeError when I first ran the ERROR option, which told me that the HTTPStatus did not have this attribute IM_A_TEAPOT, so maybe we could just throw a number(418 according to the book) instead?

Why using synchronous get in flags_asyncio.py

When downloading the flags in

async def get_flag(client: AsyncClient, cc: str) -> bytes:  4
    url = f'{BASE_URL}/{cc}/{cc}.gif'.lower()
    resp = await client.get(url, timeout=6.1,
                                  follow_redirects=True)  5
    return resp.read()  6

it seems that we read the response using .read() which is synchronous, why don't we

return await resp.aread()

instead?

The explanation seems to be for client.get() where it is doing network IO asynchrnously provided by httpx. But not for read()

6

    Network I/O operations are implemented as coroutine methods, so they are driven asynchronously by the asyncio event loop.

Is the book going to change anymore?

Hi @ramalho
I wanted to know at which stage the book is. Is it finished and revised? Cause the early access badge is gone on the book's page on O'Rielly's website.

Bug in flags2_asyncio.py (and flags3_asyncio.py) reporting

First, let me thank you very much for this great book! Fluent Python is very pedagogical, informative and quite a pleasure to read.

So, I have been quite surprised to encounter a bug when running flags2_asyncio.py, in the error accounting in the supervisor function. After a first error happens, all downloads are counted as erroneous, even when they are in fact successful. This is because the "error" variable is never reset after entering the for loop.
This can be fixed simply by adding "else: error = None" at the end of the try/except (parallel to what you do in flags2_threadpool.py and flags2_asyncio_executor.py).

This bug is also present in flags3_asyncio.py.

(I am using Python3.10.)

A big thank you again!

19-coroutine/coroaverager3:grouper

In <6> annotations ："Each iteration in thisloop creates a new instance of averager; each is a generator object operating as a coroutine." and <7>:"Whenever grouper is sent a value, it’s piped ．．．, the value it returns is bound to results[key]. The while loop then proceeds to create another averager instance to consume more values."

Would I replace while True with yield?
This seems easier to understand

The code `checked_demo.py` in Chapter 24 has TypeError: 'title' type hint must be callable?

Hello,

I run the code in https://github.com/fluentpython/example-code-2e/tree/master/24-class-metaprog/checked/metaclass/checked_demo.py

#!/usr/bin/env python3

# tag::MOVIE_DEMO[]
from checkedlib import Checked

class Movie(Checked):
    title: str
    year: int
    box_office: float

if __name__ == '__main__':
    movie = Movie(title='The Godfather', year=1972, box_office=137)
    print(movie)
    print(movie.title)
    # end::MOVIE_DEMO[]

    try:
        # remove the "type: ignore" comment to see Mypy error
        movie.year = 'MCMLXXII'  # type: ignore
    except TypeError as e:
        print(e)
    try:
        blockbuster = Movie(title='Avatar', year=2009, box_office='billions')
    except TypeError as e:
        print(e)

There is a TypeError:

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Input In [2], in <cell line: 6>()
      1 #!/usr/bin/env python3
      2 
      3 # tag::MOVIE_DEMO[]
      4 from checkedlib import Checked
----> 6 class Movie(Checked):
      7     title: str
      8     year: int

File D:\books/python/0.   Fluent Python, 2nd Edition/example-code-2e/24-class-metaprog/checked/metaclass\checkedlib.py:107, in CheckedMeta.__new__(meta_cls, cls_name, bases, cls_dict)
    105 type_hints = cls_dict.get('__annotations__', {})  # <3>
    106 for name, constructor in type_hints.items():   # <4>
--> 107     field = Field(name, constructor)  # <5>
    108     cls_dict[name] = field  # <6>
    109     slots.append(field.storage_name)  # <7>

File D:\books/python/0.   Fluent Python, 2nd Edition/example-code-2e/24-class-metaprog/checked/metaclass\checkedlib.py:76, in Field.__init__(self, name, constructor)
     74 def __init__(self, name: str, constructor: Callable) -> None:
     75     if not callable(constructor) or constructor is type(None):
---> 76         raise TypeError(f'{name!r} type hint must be callable')
     77     self.name = name
     78     self.storage_name = '_' + name  # <1>

TypeError: 'title' type hint must be callable

My python version is Python 3.8.13.

Thanks

Typo on page 745

I think there is a slight typo on page 745:

Runtime error in Example A-2 of First Edition

I'm trying to run the Example A-2 (page 689) in the First Edition.
I'm not sure if it's something wrong that I'm doing.. but I read the code 10 times. I'm sure it is the same of the book.

I'm running this code as:

$python example_a_2__generate_array.py

where my Python version is 3.8.5. Then I see this error:

initial sample: 10500000 elements
Traceback (most recent call last):
  File "example_a_2__generate_array.py", line 19, in <module>
    with len(sample) < SAMPLE_LEN:
AttributeError: __enter__

Some reasoning:
I'm not reasigning the open function (as suggested here for one possible cause).

Possible solution:
Maybe line 19 should be replaced by:

if len(sample) < SAMPLE_LEN:

I tried that and got a good looking output:

initial sample: 10500000 elements
complete sample: 10500000 elements
not selected: 500000 samples
  writing not_selected.arr
selected: 10000000 samples
  writing selected.arr

And they seem to have been created correctly:

$ls -la *selected*
-rw-rw-r-- 1 lucas lucas  4000000 mai 13 14:57 not_selected.arr
-rw-rw-r-- 1 lucas lucas 80000000 mai 13 14:57 selected.arr

PS.: Thank you for the book and sharing your knowledge!

Typing for metaclass examples

Consider the example in 24-class-metaprog/factories.py (corresponds to example 24-2, page 912 in my copy)

Dog = record_factory('Dog', 'name weight owner') 
rex = Dog('Rex', 30, 'Bob')

We do not get static type checking on, e.g., rex.name

Is there an easy fix, or is this because types in metaclasses / metaprogramming are hard?

concurrency examples

hi.
so i know that you probably know this
but if there will be a 3rd edition, exapmles 19-13 and 19-12 (and other examples in chapter 19) need a rework.
e.g the mentioned examples actually fail to prove the point of multiprocessing being good for cpu bound work since the prime number example is done in less than a second
the perf_countr() actually shows 0.00
and 19-13 actually takes longer due to multiprocessing overhead

below is the result of running them on python 3.12.2 on a not very new xeon cpu with perf_counter()

python sequential.py 

Checking 20 numbers sequentially:
               2  P  0.000001s
 142702110479723  P  0.000002s
 299593572317531  P  0.000001s
3333333333333301  P  0.000001s
3333333333333333  P  0.000001s
3333335652092209  P  0.000001s
4444444444444423  P  0.000001s
4444444444444444     0.000001s
4444444488888889  P  0.000001s
5555553133149889  P  0.000001s
5555555555555503  P  0.000001s
5555555555555555  P  0.000001s
6666666666666666     0.000001s
6666666666666719  P  0.000001s
6666667141414921  P  0.000001s
7777777536340681  P  0.000001s
7777777777777753  P  0.000001s
7777777777777777  P  0.000001s
9999999999999917  P  0.000001s
9999999999999999  P  0.000001s
Total time: 0.00s

python procs.py 

Cheking 20 numbers with 16 processes:
               2  P  0.000035s
 142702110479723  P  0.000003s
 299593572317531  P  0.000002s
3333333333333301  P  0.000001s
3333333333333333  P  0.000001s
3333335652092209  P  0.000001s
4444444444444423  P  0.000001s
4444444444444444     0.000001s
4444444488888889  P  0.000001s
5555555555555503  P  0.000001s
5555555555555555  P  0.000001s
6666666666666666     0.000001s
6666666666666719  P  0.000001s
6666667141414921  P  0.000001s
7777777536340681  P  0.000001s
7777777777777753  P  0.000001s
7777777777777777  P  0.000001s
9999999999999917  P  0.000001s
9999999999999999  P  0.000001s
5555553133149889  P  0.000033s
20 checks in 0.02s

and this is the result using perf_counter_ns()

python sequential.py 

Checking 20 numbers sequentially:
               2  P 487.000000ns
 142702110479723  P 1047.000000ns
 299593572317531  P 500.000000ns
3333333333333301  P 320.000000ns
3333333333333333  P 295.000000ns
3333335652092209  P 279.000000ns
4444444444444423  P 214.000000ns
4444444444444444    296.000000ns
4444444488888889  P 215.000000ns
5555553133149889  P 200.000000ns
5555555555555503  P 222.000000ns
5555555555555555  P 247.000000ns
6666666666666666    290.000000ns
6666666666666719  P 196.000000ns
6666667141414921  P 227.000000ns
7777777536340681  P 202.000000ns
7777777777777753  P 202.000000ns
7777777777777777  P 202.000000ns
9999999999999917  P 202.000000ns
9999999999999999  P 202.000000ns
Total time: 118353.00ns

python procs.py 

Cheking 20 numbers with 16 processes:
               2  P 13804.000000ns
 142702110479723  P 2860.000000ns
 299593572317531  P 1762.000000ns
3333333333333301  P 1324.000000ns
3333333333333333  P 1270.000000ns
3333335652092209  P 1073.000000ns
4444444444444423  P 794.000000ns
4444444444444444    1200.000000ns
4444444488888889  P 810.000000ns
5555553133149889  P 890.000000ns
5555555555555503  P 753.000000ns
5555555555555555  P 995.000000ns
6666666666666666    925.000000ns
6666666666666719  P 813.000000ns
6666667141414921  P 678.000000ns
7777777536340681  P 763.000000ns
7777777777777753  P 703.000000ns
9999999999999917  P 941.000000ns
9999999999999999  P 740.000000ns
7777777777777777  P 26330.000000ns
20 checks in 18213477.00ns

How Long ?

how long should i wait for 2nd edition? its been 1 yr + already.

Example 8-17 - Different output from expected

Hello,

Can someone please help me understand what am I doing wrong?
The example 8-17 shows this expected output:

But this is what I got instead:

I also tried that in the console session (using IPython v.7.29.0):

Is it something that might be related to different python versions (mine is 3.8.10)?
Sorry if this is outdated.

Hope to hear from you,
Kind regards

Doubt in `spinner_async.py` example

Hi Luciano,

I am playing a bit with the spinner_async.py example, and I am wondering why the execution finishes normally when I comment the spinner.cancel() line in the supervisor function. I think the execution should stay in an infinite loop because I am not cancelling the coroutine and, therefore the asyncio.CancelledError exception is never raised.

async def supervisor() -> int:  # <3>
    spinner = asyncio.create_task(spin('thinking!'))  # <4>
    print(f'spinner object: {spinner}')  # <5>
    result = await slow()  # <6>
    #spinner.cancel()  # <------This is the only change <7>
    return result

I am forgetting something?, any comments to help me to understand this scenario are welcome.

Hash Mixing

In the 1st edition you mention that you use XOR to mix hashes for Vector2d since the Python documentation recommended that approach at the time. This is no longer the case; __hash__ now states

"It is advised to mix together the hash values of the components of the object that also play a part in comparison of objects by packing them into a tuple and hashing the tuple"

So the following code snippet should be changed from:

def __hash__(self):
    return hash(self.x) ^ hash(self.y)

To:

def __hash__(self):
    return hash((self.x, self.y))

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