Python 中的 format() 实际上是做什么的？

Question

I'm coming into Python3 after spending time with Ruby, R, and some Java. Immediately I've come across the format() function and I'm a little lost as to what it does. I've read Python | format() function and see that it somehow resembles this in ruby:

my_name = "Melanie"
puts "My name is #{my_name}."

Outputs: "My name is Melanie."

However, I don't understand why I can't just use a variable as above. I must be very much misunderstanding the usage of the format() function. (I'm a novice, please be gentle.)

So what does format() actually do?

Answer 1

You can definitely use a variable in the string example that you have shown, in the following manner:

my_name = "Melanie"
Output = "My name is " + my_name + "."
print(Output)

My name is Melanie.

This is the easy way, but not the most elegant.

In the above example, I have used 3 lines and created 2 variables (my_name and Output)

However, I can get the same output using just one line of code and without creating any variables, using format()

print("My name is {}.".format("Melanie"))

My name is Melanie.

Curly braces {} are used as placeholders, and the value we wish to put in the placeholders are passed as parameters into the format function.

If you have more than one placeholder in the string, python will replace the placeholders by values, in order.

Just make sure that the number of values passed as parameters to format(), is equal to the number of placeholders created in the string.

For example:

print("My name is {}, and I am {}.".format("Melanie",26))

My name is Melanie, and I am 26.

There are 3 different ways to specify placeholders and their values:

Type 1:

print("My name is {name}, and I am {age}.".format(name="Melanie", age=26))

Type 2:

print("My name is {0}, and I am {1}.".format("Melanie",26))

Type 3:

print("My name is {}, and I am {}.".format("Melanie",26))

Additionally, by using format() instead of a variable, you can:

1. Specify the data type, and
2. Add a formatting type to format the result.

For example:

print("{0:^7} has completed {1:.3f} percent of task {2}".format("Melanie",75.765367,1))

Melanie has completed 75.765 percent of task 1.

I have set the data type for the percentage field to be a float, with 3 decimals, and given a character length of 7 to the name, and center-aligned it.

The alignment codes are:

' < ':left-align text

' ^ ':center text

' > ':right-align

The format() method is helpful when you have multiple substitutions and formattings to perform on a string.

Answer 2

An example using the format function is this:

name =  Arnold
age = 5

print("{ }, { }".format(name, age))

This displays:

Arnold, 5

Answer 3

The format function is a method for string in python, it is use to add a variable to string. for example:

greetings = 'hello {0}'
visitor = input('please enter your name')
print(greetings.format(visitor))

it can also be use to pad/position string also, thisn actually align the visitor into to the greetings in 10 byte of space

greetings = 'hello {0:^10}'
visitor = input('please enter your name')
print(greetings.format(visitor))

Also, there are two type of format in python 3x: the format expression and the format function. the format expression is actually this '%' and many more on 'format'. Maybe you should check on the doc 'format' by typing "help(''.format)"