Examples of converting between bytes and strings#
Consider a few examples of working with bytes and converting bytes to string.
subprocess#
Module subprocess returns the result of command as bytes:
In [1]: import subprocess
In [2]: result = subprocess.run(['ping', '-c', '3', '-n', '8.8.8.8'],
...: stdout=subprocess.PIPE)
...:
In [3]: result.stdout
Out[3]: b'PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.\n64 bytes from 8.8.8.8: icmp_seq=1 ttl=43 time=59.4 ms\n64 bytes from 8.8.8.8: icmp_seq=2 ttl=43 time=54.4 ms\n64 bytes from 8.8.8.8: icmp_seq=3 ttl=43 time=55.1 ms\n\n--- 8.8.8.8 ping statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time 2002ms\nrtt min/avg/max/mdev = 54.470/56.346/59.440/2.220 ms\n'
If it is necessary to work with this output further you should immediately convert it to string:
In [4]: output = result.stdout.decode('utf-8')
In [5]: print(output)
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=43 time=59.4 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=43 time=54.4 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=43 time=55.1 ms
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2002ms
rtt min/avg/max/mdev = 54.470/56.346/59.440/2.220 ms
Module subprocess supports another conversion option - encoding parameter.
If you specify it when you call run() function, the result will be as a string:
In [6]: result = subprocess.run(['ping', '-c', '3', '-n', '8.8.8.8'],
...: stdout=subprocess.PIPE, encoding='utf-8')
...:
In [7]: result.stdout
Out[7]: 'PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.\n64 bytes from 8.8.8.8: icmp_seq=1 ttl=43 time=55.5 ms\n64 bytes from 8.8.8.8: icmp_seq=2 ttl=43 time=54.6 ms\n64 bytes from 8.8.8.8: icmp_seq=3 ttl=43 time=53.3 ms\n\n--- 8.8.8.8 ping statistics ---\n3 packets transmitted, 3 received, 0% packet loss, time 2003ms\nrtt min/avg/max/mdev = 53.368/54.534/55.564/0.941 ms\n'
In [8]: print(result.stdout)
PING 8.8.8.8 (8.8.8.8) 56(84) bytes of data.
64 bytes from 8.8.8.8: icmp_seq=1 ttl=43 time=55.5 ms
64 bytes from 8.8.8.8: icmp_seq=2 ttl=43 time=54.6 ms
64 bytes from 8.8.8.8: icmp_seq=3 ttl=43 time=53.3 ms
--- 8.8.8.8 ping statistics ---
3 packets transmitted, 3 received, 0% packet loss, time 2003ms
rtt min/avg/max/mdev = 53.368/54.534/55.564/0.941 ms
telnetlib#
Depending on module, conversion between strings and bytes can be performed automatically or may be required explicitly.
For example, telnetlib module must pass bytes to read_until and
write methods:
import telnetlib
import time
t = telnetlib.Telnet('192.168.100.1')
t.read_until(b'Username:')
t.write(b'cisco\n')
t.read_until(b'Password:')
t.write(b'cisco\n')
t.write(b'sh ip int br\n')
time.sleep(5)
output = t.read_very_eager().decode('utf-8')
print(output)
Method returns bytes, so penultimate line uses decode.
pexpect#
Module pexpect waits for a string as an argument and returns bytes:
In [9]: import pexpect
In [10]: output = pexpect.run('ls -ls')
In [11]: output
Out[11]: b'total 8\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 28 12:16 concurrent_futures\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 3 07:59 iterator_generator\r\n'
In [12]: output.decode('utf-8')
Out[12]: 'total 8\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 28 12:16 concurrent_futures\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 3 07:59 iterator_generator\r\n'
And it also supports encoding parameter:
In [13]: output = pexpect.run('ls -ls', encoding='utf-8')
In [14]: output
Out[14]: 'total 8\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 28 12:16 concurrent_futures\r\n4 drwxr-xr-x 2 vagrant vagrant 4096 Aug 3 07:59 iterator_generator\r\n'
Working with files#
Until now, when working with files, the following expression was used:
with open(filename) as f:
for line in f:
print(line)
But actually, when you read a file you convert bytes to a string. And default encoding was used:
In [1]: import locale
In [2]: locale.getpreferredencoding()
Out[2]: 'UTF-8'
Default encoding in file:
In [2]: f = open('r1.txt')
In [3]: f
Out[3]: <_io.TextIOWrapper name='r1.txt' mode='r' encoding='UTF-8'>
When working with files it is better to specify encoding explicitly because it may differ in different operating systems:
In [4]: with open('r1.txt', encoding='utf-8') as f:
...: for line in f:
...: print(line, end='')
...:
!
service timestamps debug datetime msec localtime show-timezone year
service timestamps log datetime msec localtime show-timezone year
service password-encryption
service sequence-numbers
!
no ip domain lookup
!
ip ssh version 2
!
Conclusion#
These examples are shown here to show that different modules can treat the issue of conversion between strings and bytes differently. And different functions and methods of these modules can expect arguments and return values of different types. However, all of these items are in documentation.