Notes on Python: String Formatting
I’ve only recently realised I’ve been formatting text in Python in a really clunky manner. The .format() string method is something that I’ve seen and even briefly used, but never stopped and got to know well enough to use more efficiently.
So, that’s what this is, a little get to know you with string.format()
In it’s simplest terms .format() takes a string that contains braces ({}), referred to as replacement fields, and replaces them with an argument.
In [1]: "Today, I'm listening to {}.".format("Blind Willie Johnson")
Out[1]: "Today, I'm listening to Blind Willie Johnson."
We can include a number of arguments…
In [2]: listening = ("Today, I'm listening to the musician {}."
...: " Their songs include: {}, {}, {}.")
In [3]: listening.format("Blind Willie Johnson",
...: "Dark Was The Night, Cold Was The Ground",
...: "If I Had My Way I'd Tear The Building Down",
...: "It's Nobody's Fault But Mine")
Out[3]: "Today, I'm listening to the musician Blind Willie Johnson. Their songs include: Dark Was The Night, Cold Was The Ground, If I Had My Way I'd Tear The Building Down, It's Nobody's Fault But Mine."
Refer to these arguments positionally…
In [5]: listening = ("Today, I'm listening to the musician {0}."
...: " Their songs include: {2}, {3}, {1}.")
In [6]: listening.format("Blind Willie Johnson",
...: "Dark Was The Night, Cold Was The Ground",
...: "If I Had My Way I'd Tear The Building Down",
...: "It's Nobody's Fault But Mine")
Out[6]: "Today, I'm listening to the musician Blind Willie Johnson. Their songs include: If I Had My Way I'd Tear The Building Down, It's Nobody's Fault But Mine, Dark Was The Night, Cold Was The Ground."
Or even use keyword arguments…
In [7]: listening = ("Today, I'm listening to the album {album}, "
....: "by musician {musician}.")
In [9]: listening.format(
...: musician="Blind Willie Johnson",
...: album="Sweeter As The Years Go By")
Out[9]: "Today, I'm listening to the album Sweeter As The Years Go By, by musician Blind Willie Johnson."
We can also take an iterable, and access it’s elements in the string. For instance, a simple list:
In [10]: songs = ["John The Revelator",
....: "Let Your Light Shine On Me",
....: "Everybody Ought To Treat A Stranger Right"]
In [11]: songs[1]
Out[11]: 'Let Your Light Shine On Me'
In [12]: "{[1]} is currently playing".format(songs)
Out[12]: 'Let Your Light Shine On Me is currently playing'
And even something a little more complex…
In [33]: todays_music = {
....: "musician": "Blind Willie Johnson",
....: "album": "Sweeter As The Years Go By",
....: "songs": ["John The Revelator",
....: "Let Your Light Shine On Me",
....: "Everybody Ought To Treat A Stranger Right"]}
In [34]: "I'm listening to the album {[album]}".format(todays_music)
Out[34]: "I'm listening to the album Sweeter As The Years Go By"
In [35]: sentence = ("I'm listening to the album {0[album]}"
...: ", by {0[musician]},"
...: " currently playing is {0[songs][1]}")
In [36]: sentence.format(todays_music)
Out[36]: "I'm listening to the album Sweeter As The Years Go By, by Blind Willie Johnson, currently playing is Let Your Light Shine On Me"
Note that when we’re accessing more than one element from the given data structure we need to explicitly refer to that data structure. If we don’t we will use up the given data structure with the first reference to it and have nothing left available to us…
In [9]: bad_sentence = ("I'm listening to the album {[album]}"
...: ", by {[musician]},")
In [10]: bad_sentence.format(todays_music)
---------------------------------------------------------------------------
IndexError Traceback (most recent call last)
<ipython-input-10-3c99705a5b66> in <module>()
----> 1 bad_sentence.format(todays_music)
IndexError: tuple index out of range
Above, we referred to our data structure by it’s positional index, but we can also use a keyword argument…
In [13]: named_sentence = ("I'm listening to the album {music[album]}"
....: ", by {music[musician]}")
In [14]: named_sentence.format(music=todays_music)
Out[14]: "I'm listening to the album Sweeter As The Years Go By, by Blind Willie Johnson"
As well as providing replacements for placeholders in strings, we can include options to specify the format of our arguments.
We can specify the fill, the minimum amount of space to be filled…
In [18]: "{0:10} : {1:30}.".format("Musician", "Blind Willie Johnson")
Out[18]: 'Musician : Blind Willie Johnson .'
Here, within the braces we put a colon (:) before our format specifiers, the number afterwards is the size of the fill.
we can also include an alignment specifier, and a fill character…
In [19]: "{0:10} : {1:>30}.".format("Musician", "Blind Willie Johnson")
Out[19]: 'Musician : Blind Willie Johnson.'
In [20]: "{0:^10} : {1:^30}.".format("Musician", "Blind Willie Johnson")
Out[20]: ' Musician : Blind Willie Johnson .'
In [21]: "{0:_^10} : {1:_^30}.".format("Musician", "Blind Willie Johnson")
Out[21]: '_Musician_ : _____Blind Willie Johnson_____.'
In [23]: "{0:[>10} : {1:]<30}.".format("Musician", "Blind Willie Johnson")
Out[23]: '[[Musician : Blind Willie Johnson]]]]]]]]]].'
In [46]: "{0:_<{width}}{1:_<{width}}.".format("Musician", "Blind Willie Johnson", width=len("Blind Willie Johnson"))
Out[46]: 'Musician____________Blind Willie Johnson.'
We can also do some interesting things with numbers. For instance, let’s translate an IP address from decimal to binary and hexidecimal:
In [27]: "{:0>8b}.{:0>8b}.{:0>8b}.{:0>8b}".format(192,168,1,255)
Out[27]: '11000000.10101000.00000001.11111111'
In [36]: "{:^#x} {:^#x} {:^#x} {:^#x}".format(192,168,1,255)
Out[36]: '0xc0 0xa8 0x1 0xff'
So, in the first example, translating to binary we specify our format as so:
{:0>8b}
{:<fill the space with zeroes><align the number to the right><create a fill of 8 characters><provide the output in binary>}
and in the second, translating to hex:
{:^#x}
{:<center align><select alternate form of output><hex output>}
.format() is a pretty useful tool, better than sticking \t’s in your strings to format them, as I’ve been known to do :|
References: