The document discusses file input/output (I/O) operations in programming, particularly in Python. It covers various aspects such as file opening modes, data abstraction, and character encoding while providing examples of reading and writing data to files. The document emphasizes the simplicity of file I/O while highlighting potential complexities like encoding issues.

1. File-I/O
ist doch ganz einfach, oder?
Christian Kauhaus · kc@gocept.com
FLYING CIRCUS
let
y
yo u r w e b ap p f l

2. flyingcircus.io

3. with open(’users.json’, ’w’) as f:
json.dump(userdata, f)

6. Abstraktion

7. gut so

8. (meistens)

10. Kernelspace
Userspace
open(filename, mode,
Codec
encoding=...,
Universal newline
newline=...,
Buffer
Cache(s)
Filesystem
Disk
buffering=...)
os.open(filename, flags,
mode)

11. Atomarität

12. Illusion:
I/O geschieht
in einem Stück

13. with open(filename, ’wb’) as f:
f.write(data)
open("out", O_WRONLY|O_CREAT|O_TRUNC, 0666)
write(6, "1628 0
0
1"..., 1572864)
write(6, "232210H242276vn"..., 159416)
close(6)
=
=
=
=
6
1572864
159416
0

14. os.listdir(’/usr/lib’)
openat(AT_FDCWD, "/usr/lib", O_RDONLY|O_NONBLOCK|
O_DIRECTORY|O_CLOEXEC) = 3
getdents(3, /* 813 entries */, 32768)
= 32728
getdents(3, /* 809 entries */, 32768)
= 32744
getdents(3, /* 811 entries */, 32768)
= 32720
getdents(3, /* 657 entries */, 32768)
= 26424
getdents(3, /* 0 entries */, 32768)
= 0
close(3)
= 0

15. Problem?

16. with open(filename, ’rb’) as f:
data = f.read()
with open(filename, ’wb’) as f:
f.write(process(data))

17. with open(filename, ’rb’) as f:
data = f.read()
with tempfile.NamedTemporaryFile(
’wb’, dir=os.path.dirname(filename)) as t:
t.write(process(data))
tempname = t.name
os.rename(tempname, filename)

19. Persistenz

20. Illusion:
I/O greift direkt
auf die Disk zu

21. >>> f = open(’/tmp/out’, ’w’)
>>> print(’hello world’, file=f)
>>> os.system(’cat /tmp/out’)
0
>>> f.close()
>>> os.system(’cat /tmp/out’)
hello world
0

22. $ python write.py
$ ls -l out
-rw-r--r-- 1 ck users 5851 Okt 12 11:49 out
# system crash, reboot
$ ls -l out
-rw-r--r-- 1 ck users 0 Okt 12 11:49 out

23. Buffer leeren
Cache leeren

24. with open(filename, ’wb’) as f:
f.write(data)
f.flush()
os.fsync(f)

26. Text-Dateien

27. Illusion:
Dateien
enthalten
UnicodeCodepoints

28. >>> with open(filename) as f:
...
f.read()
UnicodeDecodeError: ’ascii’ codec can’t decode byte
0xc3 in position 1: ordinal not in range(128)

29. Implizite Codierung

30. # enctest.py
print(’preferred encoding:’,
locale.getpreferredencoding())
with open(’preferred’, ’w’) as f:
f.write(’hëllon’)
print(’default encoding’,
sys.getdefaultencoding())
with open(’default’, ’wb’) as f:
f.write(’hëllon’.encode())

31. $ export LC_ALL=de_DE # latin1 encoding
$ python3.2 enctest.py
preferred encoding: ISO-8859-1
default encoding utf-8
$ ls -l preferred default
-rw-r--r-- 1 ck users 6 Okt 12 12:52 preferred
-rw-r--r-- 1 ck users 7 Okt 12 12:52 default

32. explizit

33. with open(filename, ’w’, encoding=’utf-8’) as f:
f.write(data)

34. Fazit

35. File-I/O ist einfach

36. Grenzen der
Abstraktion kennen

38. Fragen?

39. Bildnachweis
© zweiwest.ch
flickr.com/teachernz
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flickr.com/iaea_imagebank
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flickr.com/atlantica
CC BY
flickr.com/jasoneppink
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