Editing 2to3 (section)

= str vs. bytes =

This can be the most tricky part. In Python 3, all strings are unicode, and <code>unicode</code> was renamed to <code>str</code> and <code>str</code> to <code>bytes</code>. Also, Python 2 allowed mixing of <code>unicode</code> and <code>bytes</code> and automatically encoded/decoded between them when needed. Python 3 doesn't do that, you have to encode and decode yourself, and pay attention to binary vs. text when writing files.

{| class="wikitable"
!
! Python 2
! Python 3
! Literals (2 and 3 compatible)
|-
! Unicode type
| unicode
| str
| u"..."
|-
! Binary type
| str (alias bytes)
| bytes
| b"..."
|}

Code examples that works in Python 2 and 3:

<syntaxhighlight lang="python">
text   = u"Greek letter alpha: \u03B1"
binary = u"Greek letter alpha: \u03B1".encode("utf-8")
binary = b"Greek letter alpha: \xce\xb1"
text   = b"Greek letter alpha: \xce\xb1".decode("utf-8")
</syntaxhighlight>

Use binary mode when reading or writing bytes from/to files:

<syntaxhighlight lang="python">
# text mode
with open("file.txt", "w") as f:
    f.write(u"Hello World\n")
    f.write(b"Hello World\n".decode("utf-8"))

# binary mode
with open("file.bin", "wb") as f:
    f.write(b"Hello World\n")
    f.write(u"Hello World\n".encode("utf-8"))
</syntaxhighlight>

The [https://docs.python.org/3/library/subprocess.html subprocess] modules opens streams in binary mode, unless you pass <code>universal_newlines=True</code>:

<syntaxhighlight lang="python">
# text mode
p = subprocess.Popen(['echo', 'Hello World'],
        universal_newlines=True,
        stdout=subprocess.PIPE)
text = p.stdout.read()

# binary mode
p = subprocess.Popen(['echo', 'Hello World'],
        stdout=subprocess.PIPE)
binary = p.stdout.read()
</syntaxhighlight>