Skip to content
Navigation Menu
{{ message }}
forked from zedr/clean-code-python
-
Notifications
You must be signed in to change notification settings - Fork 0
Expand file tree
/
Copy pathREADME.md
More file actions
1541 lines (1097 loc) · 39.4 KB
/
Copy pathREADME.md
File metadata and controls
1541 lines (1097 loc) · 39.4 KB
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
# clean-code-python
[](https://travis-ci.com/zedr/clean-code-python)
[](https://www.python.org/download/releases/3.8.3/)
## Table of Contents
1. [Introduction](#introduction)
2. [Variables](#variables)
3. [Functions](#functions)
5. [Classes](#classes)
1. [S: Single Responsibility Principle (SRP)](#single-responsibility-principle-srp)
2. [O: Open/Closed Principle (OCP)](#openclosed-principle-ocp)
3. [L: Liskov Substitution Principle (LSP)](#liskov-substitution-principle-lsp)
4. [I: Interface Segregation Principle (ISP)](#interface-segregation-principle-isp)
5. [D: Dependency Inversion Principle (DIP)](#dependency-inversion-principle-dip)
6. [Don't repeat yourself (DRY)](#dont-repeat-yourself-dry)
7. [Translations](#translation)
## Introduction
Software engineering principles, from Robert C. Martin's book
[*Clean Code*](https://www.amazon.com/Clean-Code-Handbook-Software-Craftsmanship/dp/0132350882),
adapted for Python. This is not a style guide. It's a guide to producing
readable, reusable, and refactorable software in Python.
Not every principle herein has to be strictly followed, and even fewer will be universally
agreed upon. These are guidelines and nothing more, but they are ones codified over many
years of collective experience by the authors of *Clean Code*.
Inspired from [clean-code-javascript](https://github.com/ryanmcdermott/clean-code-javascript)
Targets Python3.7+
## **Variables**
### Use meaningful and pronounceable variable names
**Bad:**
```python
import datetime
ymdstr = datetime.date.today().strftime("%y-%m-%d")
```
Additionally, there's no need to add the type of the variable (str) to its name.
**Good**:
```python
import datetime
current_date: str = datetime.date.today().strftime("%y-%m-%d")
```
**[⬆ back to top](#table-of-contents)**
### Use the same vocabulary for the same type of variable
**Bad:**
Here we use three different names for the same underlying entity:
```python
def get_user_info(): pass
def get_client_data(): pass
def get_customer_record(): pass
```
**Good**:
If the entity is the same, you should be consistent in referring to it in your functions:
```python
def get_user_info(): pass
def get_user_data(): pass
def get_user_record(): pass
```
**Even better**
Python is (also) an object oriented programming language. If it makes sense, package the functions together with the concrete implementation
of the entity in your code, as instance attributes, property methods, or methods:
```python
from typing import Union, Dict
class Record:
pass
class User:
info : str
@property
def data(self) -> Dict[str, str]:
return {}
def get_record(self) -> Union[Record, None]:
return Record()
```
**[⬆ back to top](#table-of-contents)**
### Use searchable names
We will read more code than we will ever write. It's important that the code we do write is
readable and searchable. By *not* naming variables that end up being meaningful for
understanding our program, we hurt our readers.
Make your names searchable.
**Bad:**
```python
import time
# What is the number 86400 for again?
time.sleep(86400)
```
**Good**:
```python
import time
# Declare them in the global namespace for the module.
SECONDS_IN_A_DAY = 60 * 60 * 24
time.sleep(SECONDS_IN_A_DAY)
```
**[⬆ back to top](#table-of-contents)**
### Use explanatory variables
**Bad:**
```python
import re
address = "One Infinite Loop, Cupertino 95014"
city_zip_code_regex = r"^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$"
matches = re.match(city_zip_code_regex, address)
if matches:
print(f"{matches[1]}: {matches[2]}")
```
**Not bad**:
It's better, but we are still heavily dependent on regex.
```python
import re
address = "One Infinite Loop, Cupertino 95014"
city_zip_code_regex = r"^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$"
matches = re.match(city_zip_code_regex, address)
if matches:
city, zip_code = matches.groups()
print(f"{city}: {zip_code}")
```
**Good**:
Decrease dependence on regex by naming subpatterns.
```python
import re
address = "One Infinite Loop, Cupertino 95014"
city_zip_code_regex = r"^[^,\\]+[,\\\s]+(?P<city>.+?)\s*(?P<zip_code>\d{5})?$"
matches = re.match(city_zip_code_regex, address)
if matches:
print(f"{matches['city']}, {matches['zip_code']}")
```
**[⬆ back to top](#table-of-contents)**
### Avoid Mental Mapping
Don’t force the reader of your code to translate what the variable means.
Explicit is better than implicit.
**Bad:**
```python
seq = ("Austin", "New York", "San Francisco")
for item in seq:
#do_stuff()
#do_some_other_stuff()
# Wait, what's `item` again?
print(item)
```
**Good**:
```python
locations = ("Austin", "New York", "San Francisco")
for location in locations:
#do_stuff()
#do_some_other_stuff()
# ...
print(location)
```
**[⬆ back to top](#table-of-contents)**
### Don't add unneeded context
If your class/object name tells you something, don't repeat that in your
variable name.
**Bad:**
```python
class Car:
car_make: str
car_model: str
car_color: str
```
**Good**:
```python
class Car:
make: str
model: str
color: str
```
**[⬆ back to top](#table-of-contents)**
### Use default arguments instead of short circuiting or conditionals
**Tricky**
Why write:
```python
import hashlib
def create_micro_brewery(name):
name = "Hipster Brew Co." if name is None else name
slug = hashlib.sha1(name.encode()).hexdigest()
# etc.
```
... when you can specify a default argument instead? This also makes it clear that
you are expecting a string as the argument.
**Good**:
```python
import hashlib
def create_micro_brewery(name: str = "Hipster Brew Co."):
slug = hashlib.sha1(name.encode()).hexdigest()
# etc.
```
**[⬆ back to top](#table-of-contents)**
## **Functions**
### Function arguments (2 or fewer ideally)
Limiting the amount of function parameters is incredibly important because it makes
testing your function easier. Having more than three leads to a combinatorial explosion
where you have to test tons of different cases with each separate argument.
Zero arguments is the ideal case. One or two arguments is ok, and three should be avoided.
Anything more than that should be consolidated. Usually, if you have more than two
arguments then your function is trying to do too much. In cases where it's not, most
of the time a higher-level object will suffice as an argument.
**Bad:**
```python
def create_menu(title, body, button_text, cancellable):
pass
```
**Java-esque**:
```python
class Menu:
def __init__(self, config: dict):
self.title = config["title"]
self.body = config["body"]
# ...
menu = Menu(
{
"title": "My Menu",
"body": "Something about my menu",
"button_text": "OK",
"cancellable": False
}
)
```
**Also good**
```python
class MenuConfig:
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig) -> None:
title = config.title
body = config.body
# ...
config = MenuConfig()
config.title = "My delicious menu"
config.body = "A description of the various items on the menu"
config.button_text = "Order now!"
# The instance attribute overrides the default class attribute.
config.cancellable = True
create_menu(config)
```
**Fancy**
```python
from typing import NamedTuple
class MenuConfig(NamedTuple):
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig):
title, body, button_text, cancellable = config
# ...
create_menu(
MenuConfig(
title="My delicious menu",
body="A description of the various items on the menu",
button_text="Order now!"
)
)
```
**Even fancier**
```python
from dataclasses import astuple, dataclass
@dataclass
class MenuConfig:
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool = False
def create_menu(config: MenuConfig):
title, body, button_text, cancellable = astuple(config)
# ...
create_menu(
MenuConfig(
title="My delicious menu",
body="A description of the various items on the menu",
button_text="Order now!"
)
)
```
**Even fancier, Python3.8+ only**
```python
from typing import TypedDict
class MenuConfig(TypedDict):
"""A configuration for the Menu.
Attributes:
title: The title of the Menu.
body: The body of the Menu.
button_text: The text for the button label.
cancellable: Can it be cancelled?
"""
title: str
body: str
button_text: str
cancellable: bool
def create_menu(config: MenuConfig):
title = config["title"]
# ...
create_menu(
# You need to supply all the parameters
MenuConfig(
title="My delicious menu",
body="A description of the various items on the menu",
button_text="Order now!",
cancellable=True
)
)
```
**[⬆ back to top](#table-of-contents)**
### Functions should do one thing
This is by far the most important rule in software engineering. When functions do more
than one thing, they are harder to compose, test, and reason about. When you can isolate
a function to just one action, they can be refactored easily and your code will read much
cleaner. If you take nothing else away from this guide other than this, you'll be ahead
of many developers.
**Bad:**
```python
from typing import List
class Client:
active: bool
def email(client: Client) -> None:
pass
def email_clients(clients: List[Client]) -> None:
"""Filter active clients and send them an email.
"""
for client in clients:
if client.active:
email(client)
```
**Good**:
```python
from typing import List
class Client:
active: bool
def email(client: Client) -> None:
pass
def get_active_clients(clients: List[Client]) -> List[Client]:
"""Filter active clients.
"""
return [client for client in clients if client.active]
def email_clients(clients: List[Client]) -> None:
"""Send an email to a given list of clients.
"""
for client in get_active_clients(clients):
email(client)
```
Do you see an opportunity for using generators now?
**Even better**
```python
from typing import Generator, Iterator
class Client:
active: bool
def email(client: Client):
pass
def active_clients(clients: Iterator[Client]) -> Generator[Client, None, None]:
"""Only active clients"""
return (client for client in clients if client.active)
def email_client(clients: Iterator[Client]) -> None:
"""Send an email to a given list of clients.
"""
for client in active_clients(clients):
email(client)
```
**[⬆ back to top](#table-of-contents)**
### Function names should say what they do
**Bad:**
```python
class Email:
def handle(self) -> None:
pass
message = Email()
# What is this supposed to do again?
message.handle()
```
**Good:**
```python
class Email:
def send(self) -> None:
"""Send this message"""
message = Email()
message.send()
```
**[⬆ back to top](#table-of-contents)**
### Functions should only be one level of abstraction
When you have more than one level of abstraction, your function is usually doing too
much. Splitting up functions leads to reusability and easier testing.
**Bad:**
```python
# type: ignore
def parse_better_js_alternative(code: str) -> None:
regexes = [
# ...
]
statements = code.split('\n')
tokens = []
for regex in regexes:
for statement in statements:
pass
ast = []
for token in tokens:
pass
for node in ast:
pass
```
**Good:**
```python
from typing import Tuple, List, Dict
REGEXES: Tuple = (
# ...
)
def parse_better_js_alternative(code: str) -> None:
tokens: List = tokenize(code)
syntax_tree: List = parse(tokens)
for node in syntax_tree:
pass
def tokenize(code: str) -> List:
statements = code.split()
tokens: List[Dict] = []
for regex in REGEXES:
for statement in statements:
pass
return tokens
def parse(tokens: List) -> List:
syntax_tree: List[Dict] = []
for token in tokens:
pass
return syntax_tree
```
**[⬆ back to top](#table-of-contents)**
### Don't use flags as function parameters
Flags tell your user that this function does more than one thing. Functions
should do one thing. Split your functions if they are following different code
paths based on a boolean.
**Bad:**
```python
from tempfile import gettempdir
from pathlib import Path
def create_file(name: str, temp: bool) -> None:
if temp:
(Path(gettempdir()) / name).touch()
else:
Path(name).touch()
```
**Good:**
```python
from tempfile import gettempdir
from pathlib import Path
def create_file(name: str) -> None:
Path(name).touch()
def create_temp_file(name: str) -> None:
(Path(gettempdir()) / name).touch()
```
**[⬆ back to top](#table-of-contents)**
### Avoid side effects
A function produces a side effect if it does anything other than take a value in
and return another value or values. For example, a side effect could be writing
to a file, modifying some global variable, or accidentally wiring all your money
to a stranger.
Now, you do need to have side effects in a program on occasion - for example, like
in the previous example, you might need to write to a file. In these cases, you
should centralize and indicate where you are incorporating side effects. Don't have
several functions and classes that write to a particular file - rather, have one
(and only one) service that does it.
The main point is to avoid common pitfalls like sharing state between objects
without any structure, using mutable data types that can be written to by anything,
or using an instance of a class, and not centralizing where your side effects occur.
If you can do this, you will be happier than the vast majority of other programmers.
**Bad:**
```python
# type: ignore
# This is a module-level name.
# It's good practice to define these as immutable values, such as a string.
# However...
fullname = "Ryan McDermott"
def split_into_first_and_last_name() -> None:
# The use of the global keyword here is changing the meaning of the
# the following line. This function is now mutating the module-level
# state and introducing a side-effect!
global fullname
fullname = fullname.split()
split_into_first_and_last_name()
# MyPy will spot the problem, complaining about 'Incompatible types in
# assignment: (expression has type "List[str]", variable has type "str")'
print(fullname) # ["Ryan", "McDermott"]
# OK. It worked the first time, but what will happen if we call the
# function again?
```
**Good:**
```python
from typing import List, AnyStr
def split_into_first_and_last_name(name: AnyStr) -> List[AnyStr]:
return name.split()
fullname = "Ryan McDermott"
name, surname = split_into_first_and_last_name(fullname)
print(name, surname) # => Ryan McDermott
```
**Also good**
```python
from dataclasses import dataclass
@dataclass
class Person:
name: str
@property
def name_as_first_and_last(self) -> list:
return self.name.split()
# The reason why we create instances of classes is to manage state!
person = Person("Ryan McDermott")
print(person.name) # => "Ryan McDermott"
print(person.name_as_first_and_last) # => ["Ryan", "McDermott"]
```
**[⬆ back to top](#table-of-contents)**
## **Classes**
### **Single Responsibility Principle (SRP)**
Robert C. Martin writes:
> A class should have only one reason to change.
"Reasons to change" are, in essence, the responsibilities managed by a class or
function.
In the following example, we create an HTML element that represents a comment with
the version of the document:
**Bad**
```python
from importlib import metadata
class VersionCommentElement:
"""An element that renders an HTML comment with the program's version number
"""
def get_version(self) -> str:
"""Get the package version"""
return metadata.version("pip")
def render(self) -> None:
print(f'<!-- Version: {self.get_version()} -->')
VersionCommentElement().render()
```
This class has two responsibilities:
- Retrieve the version number of the Python package
- Render itself as an HTML element
Any change to one or the other carries the risk of impacting the other.
We can rewrite the class and decouple these responsibilities:
**Good**
```python
from importlib import metadata
def get_version(pkg_name:str) -> str:
"""Retrieve the version of a given package"""
return metadata.version(pkg_name)
class VersionCommentElement:
"""An element that renders an HTML comment with the program's version number
"""
def __init__(self, version: str):
self.version = version
def render(self) -> None:
print(f'<!-- Version: {self.version} -->')
VersionCommentElement(get_version("pip")).render()
```
The result is that the class only needs to take care of rendering itself. It
receives the version text during instantiation and this text is generated by
calling a separate function, `get_version()`. Changing the class has no
impact on the other, and vice-versa, as long as the contract between them does
not change, i.e. the function provides a string and the class `__init__` method
accepts a string.
As an added bonus, the `get_version()` is now reusable elsewhere.
### **Open/Closed Principle (OCP)**
> “Incorporate new features by extending the system, not by making
> modifications (to it)”,
> Uncle Bob.
Objects should be open for extension, but closed to modification. It should be
possible to augment the functionality provided by an object (for example, a class)
without changing its internal contracts. An object can enable this when it
is designed to be extended cleanly.
In the following example, we try to implement a simple web framework that
handles HTTP requests and returns responses. The `View` class has a single
method `.get()` that will be called when the HTTP server will receive a GET
request from a client.
`View` is intentionally simple and returns `text/plain` responses. We would
also like to return HTML responses based on a template file, so we subclass it
using the `TemplateView` class.
**Bad**
```python
from dataclasses import dataclass
@dataclass
class Response:
"""An HTTP response"""
status: int
content_type: str
body: str
class View:
"""A simple view that returns plain text responses"""
def get(self, request) -> Response:
"""Handle a GET request and return a message in the response"""
return Response(
status=200,
content_type='text/plain',
body="Welcome to my web site"
)
class TemplateView(View):
"""A view that returns HTML responses based on a template file."""
def get(self, request) -> Response:
"""Handle a GET request and return an HTML document in the response"""
with open("index.html") as fd:
return Response(
status=200,
content_type='text/html',
body=fd.read()
)
```
The `TemplateView` class has modified the internal behaviour of its parent
class in order to enable the more advanced functionality. In doing so,
it now relies on the `View` to not change the implementation of the `.get()`
method, which now needs to be frozen in time. We cannot introduce, for example,
some additional checks in all our `View`-derived classes because the behaviour
is overridden in at least one subtype and we will need to update it.
Let's redesign our classes to fix this problem and let the `View` class be
extended (not modified) cleanly:
**Good**
```python
from dataclasses import dataclass
@dataclass
class Response:
"""An HTTP response"""
status: int
content_type: str
body: str
class View:
"""A simple view that returns plain text responses"""
content_type = "text/plain"
def render_body(self) -> str:
"""Render the message body of the response"""
return "Welcome to my web site"
def get(self, request) -> Response:
"""Handle a GET request and return a message in the response"""
return Response(
status=200,
content_type=self.content_type,
body=self.render_body()
)
class TemplateView(View):
"""A view that returns HTML responses based on a template file."""
content_type = "text/html"
template_file = "index.html"
def render_body(self) -> str:
"""Render the message body as HTML"""
with open(self.template_file) as fd:
return fd.read()
```
Note that we did need to override the `render_body()` in order to change the
source of the body, but this method has a single, well defined responsibility
that **invites subtypes to override it**. It is designed to be extended by its
subtypes.
Another good way to use the strengths of both object inheritance and object
composition is to use [Mixins](https://docs.djangoproject.com/en/4.1/topics/class-based-views/mixins/).
Mixins are bare-bones classes that are meant to be used exclusively with other
related classes. They are "mixed-in" with the target class using multiple
inheritance, in order to change the target's behaviour.
A few rules:
- Mixins should always inherit from `object`
- Mixins always come before the target class, e.g. `class Foo(MixinA, MixinB, TargetClass): ...`
**Also good**
```python
from dataclasses import dataclass, field
from typing import Protocol
@dataclass
class Response:
"""An HTTP response"""
status: int
content_type: str
body: str
headers: dict = field(default_factory=dict)
class View:
"""A simple view that returns plain text responses"""
content_type = "text/plain"
def render_body(self) -> str:
"""Render the message body of the response"""
return "Welcome to my web site"
def get(self, request) -> Response:
"""Handle a GET request and return a message in the response"""
return Response(
status=200,
content_type=self.content_type,
body=self.render_body()
)
class TemplateRenderMixin:
"""A mixin class for views that render HTML documents using a template file
Not to be used by itself!
"""
template_file: str = ""
def render_body(self) -> str:
"""Render the message body as HTML"""
if not self.template_file:
raise ValueError("The path to a template file must be given.")
You can’t perform that action at this time.