String Formatting

Putting Strings Together

We've already learned one way to put together strings. What is it? →

There's Also String Formatting

String formatting allows you to put placeholders in your string where values should go. One way to this is to use the % operator (there's a more modern syntax, but you'll still see this a lot):

• the % sign signals that the string on the left will be populated by the comma separated values enclosed in parentheses on the right
• %s will represent a placeholder in your string

s = "string"
print("this is a %s" % (s))

Multiple Values

• more than one %s in the string
• each substitute value separated by comma
• the original string can be a variable with a value of a string with placeholders

lyrics = "you can get with %s or you can get with %s"
s1 = "this"
s2 = "that"
print(lyrics % (s1, s2))

String Formatting

What was it again?

# note the use of variables... 
# string formatting doesn't have to consist of string literals!
# values don't have to be strings!?
greeting = "Hi.  My name is %s and I have %s %s"
name = "Joe"
number_of_pets = 314
pet = "aardvarks"
result = greeting % (name, number_of_pets, pet)
print(result)

A Closer Look at String Formatting

• we passed in a non-string type, and it didn't complain
• the placeholder, %s, signifies a string substitution
• str is called on the values (convenient!)
• there are other placeholders
  • like %i or %f, they represent other types, like int and float
  • but they don't necessarily call a conversion function
  • we'll just stick with %s right now

When to Use String Formatting

• less cumbersome than string concatenation
• less error prone! why?
• however, it can get complicated
  • there are even more features to string formatting!
    • supporting / restricting to other data types
    • formatting floats
  • sufficient to just get the basics down for our simple use-cases

So, What Does print() Do?

Both string concatenation and string formatting evaluate to a value - specifically a string! That means:

• you can assign it to a variable, etc!
• but for print… the result is None

As for print()

• it just prints out strings; it returns nothing
• using multiple parameters in print let's you skip the construction step
• (again) but it doesn't give you back a value

Greetings (Revisited)

Write a program that asks for the user's name. The program will print out "Hello (name)" using string formatting. →

What's your name?
>Joe
Hello Joe!

name = input('What\'s your name?\n>')
print("Hello %s" % (name))

And Mistakes Were Made

• string concatenation
  • adding a numeric type (we saw this earlier) →
  • missing quotes, missing plus symbol, etc. →
• string formatting
  • not enough arguments (more placeholders than values) →
  • not all arguments converted (more values than placeholders) →