First, avoid usign the identifier str. Second, you can put placeholders in strings using two methods of string formatting:

Old style

The "old" style uses C-style string formatting syntax, and "modulo" operation on the string to do the actual insertion. You can pass multiple replacements as a tuple:

s = "foo%sbaz" # expects a string
print(s%"bar")
s2 = "foo%s%d"
print(s2%("bar", 2))

New style

The "new" style uses a generic {} which can be filled using the str.format() method. Multiple replacements are passed as a unzipped tuple, i.e. as mutiple arguments:

s = "foo{}baz" # can be "anything"
print(s.format("bar"))
s2 = "foo{}{}"
print(s2.format("bar", 2))

This site might come handy as a reference.

"It does{cond} contain the the thing.".format(cond="" if condition else " not")

Thought I'd add this because it's been a feature since the question was asked, the question still pops up early in google results, and this method is built directly into the python syntax (no imports or custom classes required). It's a simple shortcut conditional statement. They're intuitive to read (when kept simple) and it's often helpful that they short-circuit.