print(“str”, a) 和 print(“str” + a) 的区别

Question

I am a beginner with python3 and I use a lot print or the logging module to follow the code on the console. A simple example below: what's the difference between:

number = "seven"
print("I cooked " , number , " dishes")

and

number = "seven"
print("I cooked " + number + " dishes")

Answer 1

The print statement supports multiple ways of parsing values.

number = 'seven'

Examples: Different style of adding argument in print statement

print("I cooked " , number , " dishes")

C-Style formatting (old):

print("I cooked %s dishes" % number)

C-Style formatting (new) using fomat :

print("I cooked {} dishes ".format(number))

f-string style

print(f"I cooked {number} dishes")

String concatenating:

print("I cooked " + number + " dishes")

You don't necessary to stick with one style. You have various options of doing the same.

Answer 2

Internally the difference is that this example (no + sign):

number = "seven"
print("I cooked " , number , " dishes")

Is printing 3 separate string objects. "I cooked " , is object 1, number is object 2, and " dishes" is object 3. So this has 3 objects total.

In the second example (with the + sign):

number = "seven"
print("I cooked " + number + " dishes")

the 3 separate strings are first being concatted into 1 new string object before being printed to stdout. So this example has 4 objects total.

Answer 3

The operator + can be only used on strings. The operator , can be used on any type, and adds a space before automatically. In addition, + can be used not only in printing but to add one string to another while , cant.

Answer 4

(Note that in the two examples you have, by simply running them will show that the results are different, as the first one will have some words separated by double spaces.)

The print() function will take in strings, each string will be printed out with a ' ' between them:

print('hello', 'world')

Output:

hello world

That is because of the keyword argument, sep . By default, sep=' ' , that is changeable by simply adding:

print('hello', 'world', sep='\n')

Output:

hello
world

The + operator will not add any separator, it will simply concatenate the strings:

print('hello' + 'world')

Output:

helloworld

Answer 5

As per PEP3105 print is considered as a function taking *args (several positional arguments).

To answer your question, the result is the same; however, your implementation is different. In the first case you give print multiple arguments to print, while in the second case you give print a concatenated string that you would like to print.

Answer 6

when using

number = "seven"
print("I cooked " , number , " dishes")

print gets 3 different objects (3 strings) as arguments, converts them to string and then prints.

However using

number = "seven"
print("I cooked " + number + " dishes")

means that first these three strings are concatenated and then passed as one object to print .

In reality, it means, that if you do for example

print('xxx' + 5 + 'yyy')

it will throw and error, as it is not possible to directly concatenate string and int types.

Also note following example:

#concatenating 3 strings and passing them as one argument to print
>>> print('xxx' + 'a' + 'yyy',sep=',')
xxxayyy

#passing 3 strings as 3 arguments to print
>>> print('xxx','a','yyy',sep=',')
xxx,a,yyy

You can notice, that in first example, although sep is used (thus it should separate 3 strigns with given separator) it does not work, because these strings are concatenated first and then passed as one argument to print . In the second example however, strings are passed as separated arguments, therefore sep=',' works, because print just knows that it should pass the separator between each given string.

Answer 7

Lets say a = "how are you" and b = "goodbye" which are 2 string variables.

1. If we do print(a, b) :

The statement will print first a then b as separate strings outputted on one line:

Output:

> how are you goodbye

2. If we do print(a + b) the statement will concatenate these two variables a and b together:

Output:

> how are yougoodbye (here there's no spacing due no white spacing in the print statement or the variables)