Cave of Python

A very common thing to want to do in Python is to create a string that has the values of variables embedded in it.

The easiest way to do this is to use Python f-strings. The string is prefixed with a lowercase or uppercase ‘F’ and then variables can be placed in the string, surrounded by curly brackets.

day = "Friday"
temperature = 25

text = f"It's {day}; the temperature is {temperature} degrees."

print(text)

It's Friday; the temperature is 25 degrees.

Using C-Style Placeholders

Another way to do the same thing is to use placeholders similar to those found in C and many other programming languages. With this method you need to use placeholders that specify the type of thing you want to embed in the string, and how to format it.

For example:

• %d: embeds a decimal integer
• %f: embeds a floating-point value
• %s: embeds a string

day = "Friday"
temperature = 25

text = "It's %s; the temperature is %d degrees." % (day, temperature)

print(text)

It's Friday; the temperature is 25 degrees.

There are lots of options we can use with these placeholders, as in other programming languages. For example, %.2f specifies that a number should be formatted as a floating-point number with two digits after the decimal point.

day = "Friday"
temperature = 25

text = "It's %s; the temperature is %.2f degrees." % (day, temperature)

print(text)

It's Friday; the temperature is 25.00 degrees.

The String Format Method

In object-oriented programming, functions that are attached to objects are known as methods. String objects in Python have a method called format, and a third option is to use this to embed values in strings. We can use empty {} brackets to specify where the values should go.

day = "Friday"
temperature = 25

text = "It's {}; the temperature is {} degrees.".format(day, temperature)

print(text)

It's Friday; the temperature is 25 degrees.

It’s also possible to use digits starting at zero to specify which argument to the format method goes where.

day = "Friday"
temperature = 25

text = "It's {1} degrees and it's {0}".format(day, temperature)

print(text)

It's 25 degrees and it's Friday

Alternatively we can even name the arguments that we supply to format.

day = "Friday"
temperature = 25

text = "It's {temp} degrees and it's {day_name}".format(day_name=day, temp=temperature)

print(text)

It's 25 degrees and it's Friday

Format String Options

It’s worth trying out all three methods, but most of the time in modern Python you’re probably going to want to use the first option, f-strings. We can supply further options after a colon in the placeholders to determine how the values are formatted.

Notice how similar this is to the C-style format specifiers.

day = "Friday"
temperature = 25

text = f"It's {day} and it's {temperature:.2f} degrees"

print(text)

It's Friday and it's 25.00 degrees

For further information, check the Python docs.