Dive Into Python 3, r916 (2010).pdf
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2011-03-13
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CHAPTER -1. WHAT’S NEW IN “DIVE INTO PYTHON
3”
❝ Isn’t this where we came in? ❞
— Pink Floyd, The Wall
-1.1. A.K.A. “THE MINUS LEVEL”
Are you already a Python programmer? Did you read the original “Dive Into Python”? Did you buy it
on paper? (If so, thanks!) Are you ready to take the plunge into Python 3? … If so, read on. (If none of that
is true, you’d be better off starting at the beginning.)
Python 3 comes with a script called 2to3. Learn it. Love it. Use it. Porting Code to Python 3 with 2to3 is a
reference of all the things that the 2to3 tool can fix automatically. Since a lot of those things are syntax
changes, it’s a good starting point to learn about a lot of the syntax changes in Python 3. (print is now a
function, `x` doesn’t work, &c.)
Case Study: Porting chardet to Python 3 documents my (ultimately successful) effort to port a non-trivial
library from Python 2 to Python 3. It may help you; it may not. There’s a fairly steep learning curve, since
you need to kind of understand the library first, so you can understand why it broke and how I fixed it. A
lot of the breakage centers around strings. Speaking of which…
Strings. Whew. Where to start. Python 2 had “strings” and “Unicode strings.” Python 3 has “bytes” and
“strings.” That is, all strings are now Unicode strings, and if you want to deal with a bag of bytes, you use
the new bytes type. Python 3 will never implicitly convert between strings and bytes, so if you’re not sure
which one you have at any given moment, your code will almost certainly break. Read the Strings chapter
for more details.
Bytes vs. strings comes up again and again throughout the book.
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