Pythonic String Formatting

Julian, Thu 02 March 2017, Learning

beginners, bestpractices, cleancode, learning, python, pythonic, tips

Formatting strings was one of the things that really hurt my head when I started learning Python. Everyone did it differently!

After doing Michael Kennedy's Write Pythonic Code Like a Seasoned Developer course, I was inspired to write about the different ways you could format a string in Python. Mainly because the method we've all deemed wrong (or at least, "least Pythonic") is the way I started off doing it!

The Terribly Unpythonic Method

Okay let's get this out of the way first. Here's how I formatted a string when I first started:

>>> country = "Australia"
>>> level = 11
>>> print("The awesomeness level of " + country + " is " + str(level) + ".")
The awesomeness level of Australia is 11.

I'll be honest, I was just glad I could get text to print! It's pretty terrible isn't it? Having to display level as a string with str() sends shivers down my spine now.

Using the String Format Operator

When Bob first showed me code that contained the string format operator I could have died. Not because it was awesome but because as a newbie, I had no idea what the heck I was even reading! Using the same variables and types:

>>> print("The awesomeness level of %s is %d." % (country, level))
The awesomeness level of Australia is 11.

What the heck are % signs doing within a string? And how are they being substituted and not being displayed as is?

It's pretty simple! What the code is doing is substituting the %s and %d for the values specified in the brackets after the string.

That is, %s is substituted with country and %d with level.

It was great until I realised that the %s was only to be used to specify a string type variable and %d to specify a decimal. (There are more options to choose from of course!).

The reason this is a pain is that you'll need to remember the type of every variable you're going to print and get the order right in your print statement. Not a huge fan.

Replacement Fields

This is my favourite one. Replacement fields!

Replacement fields expand on the format operator by taking the thinking out of the equation (always a good thing for me!):

>>> print("The awesomeness level of {} is {}.".format(country, level))
The awesomeness level of Australia is 11.

Similar to the format operator, we put the replacement fields {} where we'd like the variable output to be in the string. The difference? We no longer need to remember what the type of the variable is!

It's all taken care of by .format() which will display your variable using the appropriate format. So very cool and painless!

Pythonic goodness!

Python 3.6 f-strings

f-strings were introduced in Python 3.6 and are pretty new to me but that doesn't stop them from being awesome! Check this out:

>>> f"The awesomeness level of {country} is {level}."
'The awesomeness level of Australia is 11.'

How incredible is that? The variables are called in the string output directly within the replacement fields. You no longer need to manually call format()!

Even more Pythonic goodness!


Being quite new to Python means I'm more likely to use the latest and greatest method of formatting strings which actually makes things more difficult. I now want to go back through my old code and update it with the newer, more Pythonic methods.

Pythonic string formatting summarized

I want to hear more about how other programmers format their output strings. It's a topic that's interested me purely because of the strong opinion out there!

How do you do it?

Keep Calm and Code in Python!

-- Julian

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