Python string formatting: % vs. .format
Python 2.6 introduced the str.format() method with a slightly different syntax from the existing % operator. Which is better and for what situations?
Python format 和百分号 %的比较
To answer your first question…
just seems more sophisticated in many ways. An annoying thing about %
is also how it can either take a variable or a tuple. You’d think the following would always work:
"hi there %s" % name
yet, if name
happens to be a list: (1, 2, 3)
, it will throw a
. To guarantee that it always prints, you’d need to do
"hi there %s" % (name,) # supply the single argument as a single-item tuple
which is just ugly.
.format doesn’t have those issues. Also in the second example you gave, the .format
example is much cleaner looking.
Why would you not use it?
To answer your second question, string formatting happens at the same time as any other operation – when the string formatting expression is evaluated. And Python, not being a lazy language, evaluates expressions before calling functions, so in your
example, the expression
"some debug info: %s"%some_info
will first evaluate to, e.g.
"some debug info: roflcopters are active"
, then that string will be passed to