This question already has answers here:
Integer File Descriptor "0" in open()
(3 answers)
Closed 1 year ago.
I'm currently puzzling around this piece of code.
What exactly does
open(0)
do?
I already looked up the docs or tried to find something in the internet but no clues.
The codesnippet where this code is used:
map(abs,map(int,open(0).read().split()))
Thanks ^
0 is the file descriptor associated with stdin (1 corresponds to stdout, 2 to stderr). open takes int file descriptors as an argument, not just paths, so passing 0 is legal. This is just a somewhat obscure way to create a file object bound to stdin without needing to import sys.
The flaw with it is that, when it is closed, it will close the file descriptor 0 (since closefd=False was not passed), so sys.stdin will be closed without realizing it (though it'll probably figure it out when if someone tries to use it).
Related
This question already has answers here:
Capture stdout from a script?
(11 answers)
Closed 3 years ago.
I'm trying to get a already written line in python 3, but I haven't found any function that can read a line from the terminal. It should work something like sys.stdout.read(), or sys.stdout.readline() but this function just throws an error.
If you mean to read from the user/a pipe, then simply use input.
However, from your comments it seems like you want to be able to read from what has already been printed.
To do this, you have a few options. If you don't actually want it to display on the terminal, and you only care about certain part of the output, then you can use contextlib.redirect_stdout and contextlib.redirect_stderr. You can combine this with io.StringIO to capture the output of your application to a string. This has been discussed in the question Capture stdout from a script in Python
However, if you want to have something which provides you both a means of printing to the terminal and giving you the lines, then you will need to implement your own type which inherits from io.TextIOBase or uses io.TextIOWrapper.
Do you mean something like this?
name = input("Enter a name: ")
print(name)
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What is the python "with" statement designed for?
(11 answers)
Closed 7 years ago.
What is the difference of using:
iFile = open("filename.txt",'r')
versus
with open("filename.txt",'r') as iFile:
Is one more efficient or allow more have more methods to access? It appears to me that the with-as statement is temporary and unreferences after the following block ends.
Your first example simply opens the file and assigns the file object to a variable. You will need to manage closing the file yourself (ideally, in a try-finally block so you don't leak the file)
The second snippet uses a context manager to automatically close the file as you exit the with block, including by returning or raising an exception
This question already has answers here:
Iterating on a file doesn't work the second time [duplicate]
(4 answers)
Closed 6 years ago.
Surely this has been asked before but I cannot find the question.
As I understand it, in Python (I'm using 3.3 but this is generic to both 2.x and 3.x) you cannot iterate multiple times over an open text file and that this is due to the cursor being moved to the end and does not return to the start on the next iterable loop. Therefore it does not behave like a more typical iterable.
I was wondering how to return the cursor to the start, or at least have two for loops in succession over an open file to be read.
Thanks.
so, you're looking to rewind the file to the start again:
if your file handle is called f do it like this:
f.seek(0)
this won't work on streams, serial ports, pipes, or network sockets: only on regular files.
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Equivalent of Bash Backticks in Python [duplicate]
(11 answers)
Closed 8 years ago.
As far as you know, we can use OS console commands, (For example dir,time and format in Windows) in Python programing using os.system('TheCommand') module. But this function return the state of Operation (0 for successful and 1 for failed).
I want to know if is there any way to use the output of the commands in the next commands? I mean (For example) I run os.system('dir') and save the list of directories in a variable!
This is fairly easy to do. Here I define the working directory and the edit time of a file as variables which I used later in my script.
#!/usr/bin/env python
PWD = os.getcwd()
edit_time=os.path.getmtime(file.txt)
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Print only the message on warnings
(4 answers)
Closed 8 years ago.
When I issue a warnings.warn() warning in python, the output to stderr includes not just my warning method, but also the offending line from the source code. I would like to suppress this line from the source code, as it really detracts from expressing the warning clearly. Does anyone know how to do this?
You can monkey-patch the formatwarning function. If you look at the source, it shouldn't be too hard to write a replacement that doesn't have a line number there.