File-I/O -- ist doch ganz einfach, oder?
0 likes672 views
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.
File-I/O -- ist doch ganz einfach, oder?
- 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
- 3. with open(’users.json’, ’w’) as f: json.dump(userdata, f)
- 6. Abstraktion
- 7. gut so
- 8. (meistens)
- 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
- 24. with open(filename, ’wb’) as f: f.write(data) f.flush() os.fsync(f)
- 26. Text-Dateien
- 28. >>> with open(filename) as f: ... f.read() UnicodeDecodeError: ’ascii’ codec can’t decode byte 0xc3 in position 1: ordinal not in range(128)
- 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
- 38. Fragen?
- 39. Bildnachweis © zweiwest.ch flickr.com/teachernz CC BY-NC-SA flickr.com/iaea_imagebank CC BY-NC-ND flickr.com/atlantica CC BY flickr.com/jasoneppink CC BY-NC-SA flickr.com/seychelles88 CC BY-NC-SA