String formatting named parameters?

Question:

I know it’s a really simple question, but I have no idea how to google it.

how can I do

print '<a href="%s">%s</a>' % (my_url)

So that my_url is used twice? I assume I have to “name” the %s and then use a dict in the params, but I’m not sure of the proper syntax?


just FYI, I’m aware I can just use my_url twice in the params, but that’s not the point 🙂

Asked By: mpen

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Answers:

print '<a href="%(url)s">%(url)s</a>' % {'url': my_url}

For building HTML pages, you want to use a templating engine, not simple string interpolation.

Answered By: Mike Graham

As well as the dictionary way, it may be useful to know the following format:

print '<a href="%s">%s</a>' % (my_url, my_url)

Here it’s a tad redundant, and the dictionary way is certainly less error prone when modifying the code, but it’s still possible to use tuples for multiple insertions. The first %s is substituted for the first element in the tuple, the second %s is substituted for the second element in the tuple, and so on for each element in the tuple.

Answered By: Ponkadoodle

In Python 2.6+ and Python 3, you might choose to use the newer string formatting method.

print('<a href="{0}">{0}</a>'.format(my_url))

which saves you from repeating the argument, or

print('<a href="{url}">{url}</a>'.format(url=my_url))

if you want named parameters.

print('<a href="{}">{}</a>'.format(my_url, my_url))

which is strictly positional, and only comes with the caveat that format() arguments follow Python rules where unnamed args must come first, followed by named arguments, followed by *args (a sequence like list or tuple) and then *kwargs (a dict keyed with strings if you know what’s good for you).
The interpolation points are determined first by substituting the named values at their labels, and then positional from what’s left.
So, you can also do this…

print('<a href="{not_my_url}">{}</a>'.format(my_url, my_url, not_my_url=her_url))

But not this…

print('<a href="{not_my_url}">{}</a>'.format(my_url, not_my_url=her_url, my_url))
Answered By: Greg Ball

You will be addicted to syntax.

Also C# 6.0, EcmaScript developers has also familier this syntax.

In [1]: print '{firstname} {lastname}'.format(firstname='Mehmet', lastname='Ağa')
Mehmet Ağa

In [2]: print '{firstname} {lastname}'.format(**dict(firstname='Mehmet', lastname='Ağa'))
Mehmet Ağa
Answered By: guneysus

Solution in Python 3.6+

Python 3.6 introduces literal string formatting, so that you can format the named parameters without any repeating any of your named parameters outside the string:

print(f'<a href="{my_url:s}">{my_url:s}</a>')

This will evaluate my_url, so if it’s not defined you will get a NameError. In fact, instead of my_url, you can write an arbitrary Python expression, as long as it evaluates to a string (because of the :s formatting code). If you want a string representation for the result of an expression that might not be a string, replace :s by !s, just like with regular, pre-literal string formatting.

For details on literal string formatting, see PEP 498, where it was first introduced.

Answered By: gerrit

Another option is to use format_map:

print('<a href="{s}">{s}</a>'.format_map({'s': 'my_url'}))
Answered By: Danny Varod

I recommend this syntax

dictionary_of_string_values = {
                               "my_text" : "go to w3schools",
                               "my_url" : "https://www.w3schools.com",
                               }

print ('<a href="{my_url}">{my_text}</a>'.format(**dictionary_of_string_values))

It is very useful when you have to format a string with lots of placeholders.

You can also make it shorter like this:

print ('<a href="{my_url}">{my_text}</a>'.format(
                             **{
                               "my_text" : "go to w3schools",
                               "my_url" : "https://www.w3schools.com",
                               }
                            )
                        )
Answered By: Tms91
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