if STREAM.isatty():

[Peter Otten]

Hongyi Zhao wrote:

> On Thu, 29 Aug 2019 16:42:44 +0100, Rhodri James wrote:
> 
>> I don't understand what's to not to understand.
>> 
>>    if condition:
>>      do_something_because_condition_is_true()
>>    else:
>>      do_something_because_condition_is_false()
>> 
>> is a perfectly normal construction.  If you mean something else, please
>> be explicit.
> 
> Let me say it more explicitly:
> 
> The original code snippet's logic is as follows:
> 
>         if STREAM.isatty():
>             p = (self.progress_template + '\r') % params
>         else:
>            [do something to reset the new value of p]
>            p = (self.progress_template + '\n') % params
>         STREAM.write(p)
>         STREAM.flush()
> 
> 
> In order to refresh the tty, the if and else must be run alternatively.
> i.e., the isatty() must return 0 and 1 periodically.
> 
> I still understand how the above code will let the isatty() return 0 and
> 1 in turn?

Perhaps a simple example can help?

$ cat checktty.py
import sys

stream = sys.stdout

if stream.isatty():
    message = "tty"
else:
    message = "no tty"
print(message, file=stream)

When you run the script it prints to the terminal:

$ python3 checktty.py 
tty

But when you redirect to a pipe or into a file:

$ python3 checktty.py | cat
no tty

$ python3 checktty.py > tmp.txt
$ cat tmp.txt
no tty