Python Format Best Practices

Question

I'm just coming up to speed with Python and had a question about best practices (or at least common practices) around using .format on a string.

My question is mostly around when you would use blank curly brackets vs. an index number vs. a name.

For example if you had a single variable that you wanted to include in a string which one would you do?

print "I {} Stack Overflow".format(var)
print "I {0} Stack Overflow".format(var)
print "I {verb} Stack Overflow".format(verb = var)

Does this change if you have multiple variables you want to include? Maybe it's OK to include {} for a single var, but never for multiple vars?

Any thoughts or insights here would be greatly appreciated.

Thanks!

• Joran Beasley: Im not sure what pep8 says ... but I dislike the empty braces ... and my favorite is method3 ... but thats probably just me
• Simeon Visser: I tend to use {} or {word}, depending on what's more readable for the task at hand.
• Niyojan: This, my friend, depends on your taste. :)

Answer 1

I don't think there are (yet) any practices established as "best" or even as "common", so you'll get a bunch of opinions (including mine:-).

I think that {named} curlies are the way to go when your format string is a variable (e.g coming from a database or config file, picked depending on user's chosen language, etc, etc) because they let you pick and choose which of the (named) arguments to format, the order, possibly repeat them, and so forth, while staying readable.

If the format string is a literal, empty curlies a la {} are least obtrusive and thus may end up most readable -- unless your format string has "too many" of them, which of course is a style judgment.

At least it's a style issue very cognate to the one you face every time you define or call a function -- how any positional arguments or parameters are "too many" for readability, should you go whole hogs to named parameters and arguments only, etc, etc. Similar considerations apply!

• Joran Beasley: "{:{}{}d}".format(2,0,3) only has 3 :P (I think I got the argument order right there ... )
• Alex Martelli: @JoranBeasley, I agree with the implication that readability can also suffer when relatively few anonymous entries are "embricated" in unfamiliar ways. It's also interesting to note that adding arg numbers to the format string would be no help to readability either -- only names on both sides might help.
• Joran Beasley: @AlexMartelli well I slightly disagree ... at least with number indexing I could tell which arguement went were :P (+1 all the same ofc because you always know your stuff :P)
• Alex Martelli: @JoranBeasley, I can easily count up to 3 open-braces in string with '{:{}{}d}' -- spelling it {0:{1}{2}d} would have negative benefit for me, it only adds noise. Now '{dat:{fil}{wid}d}'.format(dat=2, fil=0, wid=3), while obviously more verbose, does add some value wrt the args' roles.

Answer 2

print " I {} Stack Overflow.".format(var) is the correct way.

If you need multiple variable you would just place more curly brackets and add the var names separated by a comma.

print "I {} Stack Overflow. It is a great {} {}!".format(var, var1, var3)

• Joran Beasley: they are all "correct" ... I am curious where you get the info that empty braces are the preferred way over indexes or variable names
• NimbusScale: All three methods I describe above are valid, why do you say the empty curly bracket method is the "correct" way?
• Brandon Pharis: that was just how I was taught. Good to know that it can be done the other 2 ways as well. thanks
• Joran Beasley: "I {feel_about} Stack Overflow.".format(feel_about="am annoyed with")

Answer 3

I know this is an old question, but Python 3.6 brings a very cool string format, called "f-strings". It's similar to Javascript. Here is an simple example:

name = "John"
age = 25
print(f"My name is {name} and I am {age} years old.")

It only works in Python 3.6 and above, but I think it's the easiest and more readable way to format strings in Python.

Answer 4

It's ok to use empty braces for one or two variables. I would recommend using named replacement:

• for more variables
• in situations where replaced string is hard to read
• when injected var is repeated.

• NimbusScale: So you would never use the index number method?
• Daniel: I would use the index one when reusing a variable in the format process "{0} {1} {0}".format(a,b)