Python 3 interpreter prints length to standard input for every write [duplicate]

问题

There are a few questions about this kind of behaviour:

>>> import sys
>>> sys.stdout.write("aaaa")
aaaa4
>>>

I understand what's going on there. What I don't understand is what's happening in my case: regardless of which file I open, whenever I use its .write method, the length of the data is written to the console/to stdout.

>>> with open("garbage.file", "wb") as f:
...     for x in range(4):
...         f.write(b"xyz")
...
3
3
3
3
>>> with open("garbage.file", "rb") as f:
...     assert f.read() == b"xyzxyzxyzxyz"
...
>>>

However, this behaviour does not occur when I let python run it as a script:

D:\>type CON > test.py
with open("garbage.file", "wb") as f:
    f.write(b"xyz")

^Z

D:\>python test.py

D:\>type garbage.file
xyz
D:\>

This happens with any fresh Python 3.5 interpreter used on a Windows command prompt (either the "normal" cmd or the "Anaconda Prompt").

>>> import sys
>>> sys.version
'3.5.2 |Anaconda 4.1.1 (64-bit)| (default, Jul  5 2016, 11:41:13) [MSC v.1900 64 bit (AMD64)]'
>>>

I've never seen this behaviour before, and it doesn't look like it's supposed to happen either. What could be the cause? How can I resolve it?

回答1:

This seems to be a duplicate of this question: sys.stdout.write in python3 adds 11 at end of string

Which explains that .write() returns the number of characters written after it's been written to the file. Which explains why you see it in the interpreter but not the file you've created.

Edit: Examples showing the interpreter show return values and the python executable ignore them.

>>> def show(string):
...     print(string)
...     return(len(string))
... 
>>> show('foobar')
foobar
6

Now if I create a file with the exact same contents I get:

$ python show.py 
foobar

This is because the python executable doesn't show returned values whereas the interpreter does.

来源：https://stackoverflow.com/questions/39592227/python-3-interpreter-prints-length-to-standard-input-for-every-write

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python

Anaconda