Python F-string Formatting

Before Python 3.6, you had two options to format your strings:

• % formatting.
• str.format().

The F-string formatting option has been available since Python 3.6.

In this lesson, I’ll show you why the older % formatting and str.format() options to format your strings are obsolete nowadays and how the F-string formatting option makes your life much easier.

% Formatting

The %-formatting option has been available since the beginning of Python. The % operator is built into string objects. You can use this to format strings. For example:

You can use the % operator multiple times, but you’ll need to use a tuple () to supply the values you want to use to format your string:

>>> router_hostname = "R1"
>>> switch_hostname = "SW1"
>>> print("Device hostnames are %s and %s." % (router_hostname, switch_hostname))

Will show this output:

Device hostnames are R1 and SW1.

If you only want to format your strings with one or two values, the % operator works fine. However, with many values to replace, your code becomes difficult to read:

>>> vendor = "Cisco"
>>> platform = "ISR 4311"
>>> hostname = "R1"
>>> uptime = "3600"
>>> ios_version = "Gibraltar-16.12.7"
>>> print("Router %s from vendor %s has hostname %s, runs OS %s and has uptime of %s seconds." % (platform, vendor, hostname, ios_version, uptime ))

This is the output:

Router ISR 4311 from vendor Cisco has hostname R1, runs OS Gibraltar-16.12.7 and has uptime of 3600 seconds.

In the code above, you can see the problem with the % operator. Your code becomes difficult to read because you can’t really tell which value replaces what % symbol.

str.format()

Python 3 has a newer way to format strings by using .format() on a string object. Here is a quick example that looks similar to the old %-formatting method.

>>> hostname = "SW1"
>>> uptime = "5436"
>>> print("Switch {} has an uptime of {} seconds.".format(hostname, uptime))

This is the output:

Switch SW1 has an uptime of 5436 seconds.

It is possible to specify the values you want to use to format your strings within the strings:

>>>sw1_hostname = "SW1"
>>> sw1_uptime = "5436"
>>> print("Switch {hostname} has an uptime of {uptime} seconds.".format(hostname=sw1_hostname, uptime=sw1_uptime))

This is the output:

Switch SW1 has an uptime of 5436 seconds.

This makes your code easier to read than the % formatting method. However, if you have many things in your string that you want to format, your code becomes messy:

>>> r1_hostname = "R1"
>>> r1_location = "Amsterdam"
>>> r1_status = "Online"
>>> r1_uptime = "7451"
>>> print("Router {hostname} located at {location} is {status} has an uptime of {uptime} seconds.".format(hostname=r1_hostname, location=r1_location, status=r1_status, uptime=r1_uptime))

This is the output:

Router R1 located at Amsterdam is Online has an uptime of 7451 seconds.

F-Strings

F-strings are easy to work with and keep your code readable. Here is a quick example:

>>> hostname = "SW1"
>>> vendor = "Cisco"
>>> platform = "Catalyst"
>>> print(f"Switch {hostname} from {vendor} is a {platform} switch.")

This is the output:

Switch SW1 from Cisco is a Catalyst switch.

Before the string, you add an “f”. This can be a small or capital letter. You can then add whatever you want to format in between {}. A huge advantage of this way of formatting is that your code remains easy to read, even when you want to format many things in your string. For example:

>>> vendor = "Cisco"
>>> platform = "8100"
>>> hostname = "R2"
>>> uptime = "7456"
>>> ios_version = "IOS XR"
>>> print(f"Router {hostname} from vendor {vendor} called {hostname} runs {ios_version} and has uptime of {uptime} seconds.")

This is the output:

Router R2 from vendor Cisco called R2 runs IOS XR and has uptime of 7456 seconds.

Python Expressions

I used variables like “vendor” or “platform” with assigned strings in the examples above. Python evaluates f-strings at runtime, allowing you to run any valid Python expressions in a string. Here is an example:

>>> uptime = 28800
>>> print(f"Router has an uptime of {uptime / 3600} hours.")

This is the output:

Router has an uptime of 8.0 hours.

In the example above, I use an arithmetic operator (division) in the f-string.

>>> vendor = "CiScO"
>>> print(f"Router is from vendor: {vendor.title()}.")

This is the output:

Router is from vendor: Cisco.

Here is one more example where I use the title() method: