Top  | Previous | Next

String Formatting

String formatting is a somewhat minor feature of Python, but turns out to be incredibly useful in Ignition. String formatting is used to manipulate strings, specifically to insert the values of variables inside a string without a bunch of concatenation.

 

The % operator is used in Python not just for modulus, but also for string formatting. Suppose we wanted to print a weather report. We could use concatenation, like this:

 

temp = 65.8

city = "Sacramento"

windSpeed = 25

windDir = "east"

 

print city 

print "Weather: " + str(temp) + "°F, wind "+ str(windSpeed) + "mph from the "+ windDir

 

Yuck! This kind of concatenation is really a pain to write and to read. With string formatting, we could have written it like this:

 

temp = 65.8

city = "Sacramento"

windSpeed = 25

windDir = "east"

 

print "%s weather: %f°F, wind %dmph from the %s" % (city, temp, windSpeed, windDir)

 

Ah, that's much easier on the eyes. What is happening here is that the % operator is applying the variables on its right-hand side to the format string on its left-hand side. It looks for placeholders (called format specifiers) inside the format string, and replaces them with corresponding values from the variables on the right-hand side. There are various format specifiers that can be used for different types of variable types. If you actually want a % sign inside the final string, use the special format specifier: "%%"

 

Format Specifier

Meaning

%%

Inserts a % sign into the final string

%c

A single character. Value must be a string of length 1 or an integer

%d or %i

Signed integer

%f

Floating point, decimal format

%s

A String, converts the value to a string using str()

%u

Unsigned decimal

%x or %X

Unsigned hexadecimal

 

You can also put some extra information in the format specifiers between the % and the format specifier character. The most useful thing to do is to specify the number of decimal places to use to print floating point numbers. For example, "%.3f" would always put three digits after the decimal point.