python的字符串格式中的%a是什么

Question

I have a basic question regarding string formatting. I use both python 2.7 and 3+ for my coding

I understand that %s is used for characters or strings

name = "John"
print("Hello, %s!" % name) 

Hello, John!

now, if I use an integer

name = 123
print("Hello, %s!" % name)

the output is Hello, 123!

And when I do want to convert it to bytes, something like this

name = 123
print(b"Hello, %s!" % name)

in python 2.7 it throws me an error saying that %b requires bytes, or an object that implements __bytes__, not 'int'

name = 123
print(b"Hello, %a!" % name)

Is %a is explicitly used to take the bytes kind of stuff .. I couldn't find much info on %a on web.. and the old code which someone wrote extensively used it and I am debugging that .. SO need some help in understanding the above mentioned (using strings or numbers while converting to bytes )

Answer 1

%a will give the equivalent of repr(some_obj).encode('ascii', 'backslashreplace') on the interpolated value. Use cases include developing a new protocol and writing landmarks into the stream; debugging data going into an existing protocol to see if the problem is the protocol itself or bad data; a fall-back for a serialization format; or any situation where defining bytes would not be appropriate but a readable/informative representation is needed [6].

%r is included as a synonym for %a for the sole purpose of making 2/3 code bases easier to maintain. Python 3 only code use %a

https://www.python.org/dev/peps/pep-0461/

Answer 2

在打印语句中，您可以用 str(name).encode() 替换 name 以消除您遇到的错误。

Answer 3

%b is used for formatting a byte value in the print statement.

s = "hello"
s_bytes = str.encode(s)
type(s_bytes)

class 'bytes'

s_bytes

b'hello'

print(b"The text is %b" % s_bytes)

b'The text is hello'

print(b"The text is %b" % s)

Traceback (most recent call last): File "", line 1, in TypeError: %b requires a bytes-like object, or an object that implements bytes , not 'str'

The bottom line is, you have to pass a byte object when you are formatting with %b .

Answer 4

'%a' string format operator. '%a' converts any python object to a string using repr() and then hex-escapes all non-ASCII characters. The '%a' format operator generates the same string as '%r' in Python 2. Also, add '!a' conversion flags to the string.format() method and add '%A' operator to the