If you ever came across a situation where you have test your program against a very large pile of input, you have wondered that if there's any way to shortcut o it.
There are certain methods often come very handy when you have to test your program time and space complexity while processing a large input
You cant always input some large input manually , so there's the method by which you can provide input to your program using external txt file
Below is my answer to it :)
You just write simple program and then run it from command line in any platform like Windows/ Linux.
python program.py < input.txt > output.txt
<,> are redirection operators which simply redirects the stdin and stdout to input.txt and output.txt. This is the easiest way.
Alternatively, you can do the following
import sys sys.stdin=open('input.txt','r') sys.stdout=open('output.txt','w')
Alternatively you can do the following
input=open('input.txt','r')
ouput=open('ouput.txt','w')
n=input.read()
output.write(n)
I prefer method 1 as it is simple and no need of file handling and this helps a lot in Codejam, FaceBook HackerCup. Hope it helps
Related
I have two programs written in python and converted to one-file exe using auto-py-to-exe.
the first program writes to a file, which is read by the second program. The problem is when the second program wants to read the file the same time as it is being written, the code stops with a permission error.
The solutions that seemed to work are:
Using time management which is not useful in my case, since the reading and writing times are not constant.
I could check if the file is accessible, which might be a solution, however, I suppose it would raise an error if while reading the file, the writer tries to change the file.
I could use the size of the file to check if writing to the file has been finished, and then execute the reader, however, this does not seem to be both logical and pythonic!
I found some solutions using os.pipe(), but to be honest, I couldn't understand what the process does. If this is a solution, I would be glad to have it explained in simple English.
That's it. Any suggestions?
P.S: OS is windows and I am using Python 3.9
Solved:
Thanks to the replies and suggestions, I didn't know that the try except commands accept ErrorType. Thus, I solved the problem by using 'except' and 'PermissionError'. the code runs in a loop and it is checked again in a few seconds.
However, the drawback is this: the reading time should be less than the time the writer comes back to rewrite the file! In my case, as suggested by friends, I combined the two programs so they are run sequentially.
I have a collection of expert advisor (EA) scripts written in the MQL5 programming language for the stock/forex trading platform, MetaTrader5. The extension of these files is mq5. I am looking for a way to programatically run these MQL5 files from my Python script on a regular basis. The EAs do some price transformations, eventually saving a set of csv files that will later be read by my Python script to apply Machine Learning models on them.
My first natural choice was the Python API for MetaTrader5. However, according to its documentation, it "is designed for convenient and fast obtaining of exchange data via interprocessor communication directly from the MetaTrader 5 terminal" and as such, it doesn't provide the functionality I need to be able to run MQL scripts using Python.
I have found some posts here on SO (such as #1, #2) about executing non-python files using Python but those posts seemed to always come with the precondition that they already had Python code written in them, only the extension differed - this is different from my goal.
I then came across Python's subprocess module and started experimenting with that.
print(os.path.isfile(os.path.join("path/to/dir","RSIcalc.mq5")))
with open(os.path.join("path/to/dir","RSIcalc.mq5")) as f:
subprocess.run([r"C:\Program Files\MetaTrader 5\terminal64.exe", f], capture_output=True)
The print statement returns True, so the mq5 file exists in the specified location. Then the code opens the MetaTrader5 terminal but nothing else happens, the EA doesn't get executed, process finishes immediately after that.
Am I even on the right track for what I'm trying to achieve here? If yes, what might be the solution for me to run these MQL5 scripts programatically from Python?
Edit:
I use Windows 10 64-bit.
subprocess is indeed the right module for what you want to achieve. But let's look at what you're doing here:
with open(os.path.join("path/to/dir","RSIcalc.mq5")) as f
You're creating a file descriptor handle called f, which is used to write or read contents from a file. If you do print(f) you'll see that it's a python object, that converted to string looks like <_io.TextIOWrapper name='RSIcalc.mq5' mode='r' encoding='UTF-8'>. It is extremely unlikely that such a string is what you want to pass as a command-line parameter to your terminal executable, which is what happens when you include it in your call to subprocess.run().
What you likely want to do is this:
full_path = os.path.abspath(os.path.join("path/to/dir","RSIcalc.mq5"))
result = subprocess.run([r"C:\Program Files\MetaTrader 5\terminal64.exe", full_path], capture_output=True)
Now, this assumes your terminal64 can execute arbitrary scripts passed as parameters. This may or may not be true - you might need extra parameters like "-f" before passing the file path, or you might have to feed script contents through the stdin pipe (unlikely, on Windows, but who knows). That's for you to figure out, but my code above should probably be your starting point.
I don’t think you need to be passing a file object to your sub process statement. In my experience. A program will run a file when the path to the file is provided as a command line argument. Try this:
subprocess.run([r"C:\\Program Files\\MetaTrader 5\\terminal64.exe", os.path.join(“path/to/dir”, “RSIcalc.mq5”], capture_output=True)
This is the same as typing C:\Program Files\MetaTrader 5\terminal64.exe path\to\dir\RSIcalc.mq5 in your terminal.
I need to use an old-fashioned DOS/Windows executable (the source is not available). It uses two input files and produces one output file.
I have to run this several thousands times, using different input files. I wrote a simple python script looping over input files to automate this.
The problem is that this exe finishes every single run with the immortal "press Enter".
I start the script, keep the key pressed, 'returns' accumulate in the bufor and the script runs for a while producing several outputs.
Is there any more elegant way to proceed (i.e. without using the finger and staring at the monitor)?
I have already tried some obvious solutions (e.g. os.system('return'), os.system('\n')) but they do not work.
Next day edit:
#Eric, many thanks for the code, it works. I also thank others who contribute, and sorry for slopply written question and unformatted code in the comment (it was 3.30 am :)
From the information in your comment, what I think you want is something like:
import subprocess
for i in range(1, 20001):
command = "wine executable.exe input{number}.txt > output{number}.txt".format(number=i)
p = subprocess.Popen(command, stdin=subprocess.PIPE, shell=True)
# send a newline
p.communicate(input="\n")
Use Python's subprocess module and run your executable with Popen.
Then you can send "enter" to the process with communicate.
Have you tried os.system(‘\r\n’)? I think that’s the newline character on windows.
Edit: Your answer also used a forward slash instead of a backslash--definitely try the other way too, unless that’s just a typo.
My question is somewhat unique. I am currently working on a project for my computer forensics class. This project is aimed at hiding disk data from investigators. The method by which this is supposed to be achieved is by writing the bytes of a "clean" file over the "bad" file. Once overwritten, the "bad" file is deleted.
This concept sounds simple enough, but what my partner and I have observed is interesting. If we open a file in a python script, we can easily overwrite the memory associated with that file on disk (verified using dd). We can also easily delete a file using from inside the script. However, a write then delete results in no write actually taking place, only the file's removal.
This makes sense from an OS optimization standpoint. From that point, we thought it might work if we split the writing and deleting into two separate scripts, and controlled both by a third. However, it seems that even if we run the scripts as a subprocess of another script, the same thing happens. We've tried to use bash scripts for the deletion process instead of pure python, and still, nothing sticks.
This project was supposed to be a whole mess of little anti-forensics tools like this, but this particular one has captured our whole attention because of this issue. Does anyone have an idea as to why this is happening and what we can do to move forward?
We know this can be achieved in C, etc, but we want to solve this using python because of the interesting constraints it's presented.
---EDIT---
This is a snippet from our controller, it calls "ghost.py" with the associated params.
ghost.py prints the edited file names/paths to stdout.
Relevant code follows:
proc = subprocess.Popen(['python', 'ghost.py', '-c', 'good.txt', '-d','/mnt/evil.txt'], stdout=subprocess.PIPE,)
files = proc.communicate()
for i in files:
if i != None and i != "\n":
os.system("./del.sh " + i)
Using a subprocess doesn't change any interesting aspect of your design, so don't use them. You probably need os.fsync(). Try this pattern:
myfile.write('all of my good data')
myfile.flush()
os.fsync(myfile.fileno())
myfile.close()
os.remove(myfile)
Reference: https://docs.python.org/2/library/os.html#os.fsync
I am new to python and have until now written only a few programs to help with my job (I'm a sysadmin). I am writing this script now which will write the output of a MySQL query to a file. While in-between looping, I want to check for an extra condition and if the condition does not match, I want to close the file that I am writing to without saving what it has already written to the file. Like 'exit without saving'. I wrote this simple code to see if not closing the file with a close() will exit without saving, but it is creating the file with the content after I run and exit this code. So, is there a legal way in Python to exit a file without saving?
#/usr/bin/python
fo=open('tempfile.txt','a')
fo.write('This content\n')
P.S:- Python version is 2.4.3 (sorry, cannot upgrade)
There is no such concept in programming.
For the vast majority of the programming languages out there, the write command will attempt to put data directly in the file. This may or may not occur instantly for various reasons so many languages also introduce the concept of flush which will guarantee that your data is written to the file
What you want to do instead is to write all your data to a huge buffer (a string) then conditionally write or skip writing to the file.
Use the tempfile module to create your temporary, then if you need to save it you can do so explicitly using shutil.copyfileobj.
See Quick way to save a python TempFile?
Note that this is only if you absolutely need a temporary file (large amounts of data, etc.); if your contents are small then just using a stringbuffer and only writing it if you need to is a better approach.
Check for the condition before opening the file:
#/usr/bin/python
if condition():
fo=open('tempfile.txt','a')
fo.write('This content\n')
The safest way to do this is not to write to, or even open, the file until you know you want to save it. You could, for example, save what you might eventually write to a string, or list of same, so you can write them once you've decided to.