Case 1:
"{arg1} {arg2}".format(10, 20)
It will give KeyError: 'arg1'
because I didn't pass the named arguments.
Case 2:
"{arg1} {arg2}".format(arg1=10, arg2=20)
Now it will work properly because I passed the named arguments. And it prints '10 20'
Case 3:
And, If I pass wrong name it will show KeyError: 'arg1'
"{arg1} {arg2}".format(wrong=10, arg2=20)
But,
Case 4:
If I pass the named arguments in wrong order
"{arg1} {arg2}".format(arg2=10, arg1=20)
It works...
and it prints '20 10'
My question is why does it work and what's the use of named arguments in this case.
' Formatted string literals are a Python parser feature that converts f-strings into a series of string constants and expressions. They then get joined up to build the final string.
The __format__ method is responsible for interpreting the format specifier, formatting the value, and returning the resulting string. It is safe to call this function with a value of “None” (because the “None” value in Python is an object and can have methods.)
They are used for formatting strings. %s acts a placeholder for a string while %d acts as a placeholder for a number. Their associated values are passed in via a tuple using the % operator.
Named replacement fields (the {...}
parts in a format string) match against keyword arguments to the .format()
method, and not positional arguments.
Keyword arguments are like keys in a dictionary; order doesn't matter, as they are matched against a name.
If you wanted to match against positional arguments, use numbers:
"{0} {1}".format(10, 20)
In Python 2.7 and up, you can omit the numbers; the {}
replacement fields are then auto-numbered in order of appearance in the formatting string:
"{} {}".format(10, 20)
The formatting string can match against both positional and keyword arguments, and can use arguments multiple times:
"{1} {ham} {0} {foo} {1}".format(10, 20, foo='bar', ham='spam')
Quoting from the format string specification:
The field_name itself begins with an arg_name that is either a number or a keyword. If it’s a number, it refers to a positional argument, and if it’s a keyword, it refers to a named keyword argument.
Emphasis mine.
If you are creating a large formatting string, it is often much more readable and maintainable to use named replacement fields, so you don't have to keep counting out the arguments and figure out what argument goes where into the resulting string.
You can also use the **keywords
calling syntax to apply an existing dictionary to a format, making it easy to turn a CSV file into formatted output:
import csv fields = ('category', 'code', 'price', 'description', 'link', 'picture', 'plans') table_row = '''\ <tr> <td><img src="{picture}"></td> <td><a href="{link}">{description}</a> ({price:.2f})</td> </tr> ''' with open(filename, 'rb') as infile: reader = csv.DictReader(infile, fieldnames=fields, delimiter='\t') for row in reader: row['price'] = float(row['price']) # needed to make `.2f` formatting work print table_row.format(**row)
Here, picture
, link
, description
and price
are all keys in the row
dictionary, and it is much easier to see what happens when I apply the row
to the formatting string.
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