Virtual Env
https://packaging.python.org/guides/installing-using-pip-and-virtualenv/
setup virtual env
$ python3 -m virtualenv env
switch env (goto the folder)
$ source env/bin/activate
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Python Tips
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Job Scheduling
from apscheduler.schedulers.blocking import BlockingScheduler scheduler = BlockingScheduler() @scheduler.scheduled_job('cron', hour=16, minute=59, second=59) def execute_daily_task(): pass # do something scheduler.start() # start scheduler -
howto: run python script from scheduler
import sys import time from subprocess import Popen, PIPE from apscheduler.schedulers.background import BackgroundScheduler from apscheduler.schedulers.blocking import BlockingScheduler scheduler = BackgroundScheduler( {'apscheduler.job_defaults.max_instances':'2'} ) blocker = BlockingScheduler() theproc = None def run_task(): # run long process task print('run task', time.ctime()) global theproc theproc = Popen([sys.executable, "LongProcessTask.py"], stdout=PIPE) result = theproc.communicate() print(result) # run task every 30 mins @scheduler.scheduled_job('interval', minutes=30) def execute_restart(): print('restarting', time.ctime()) global theproc # if old process exist, kill it if theproc is not None: print('kill old process') theproc.kill() # run task run_task() scheduler.add_job(run_task, 'date') # run the task now scheduler.start() # blocker prevent the program from exiting blocker.start() -
howto: Print object attributes
class ObjectHelper(object): @staticmethod def stringify(obj): attrs = vars(obj) return ', '.join("%s: %s" % item for item in attrs.items()) -
# plugin framework def start(self): """Start tracker engine. """ if not self._loop.is_running(): LOGGER.info('Run event loop forever') threading.Thread(target=self._loop.run_forever).start() if not self._is_started: self._loop.call_soon_threadsafe( asyncio.ensure_future, self._start() ) LOGGER.info("Request progress tracker started") self._is_started = True return if self._is_paused: self._is_paused = False else: LOGGER.warning('Request progress tracker already started') -
howto: convert ascii text and image
https://stackoverflow.com/questions/29760402/converting-a-txt-file-to-an-image-in-python
import PIL import PIL.Image import PIL.ImageFont import PIL.ImageOps import PIL.ImageDraw PIXEL_ON = 0 # PIL color to use for "on" PIXEL_OFF = 255 # PIL color to use for "off" def main(): image = text_image('content.txt') image.show() image.save('content.png') def text_image(text_path, font_path=None): """Convert text file to a grayscale image with black characters on a white background. arguments: text_path - the content of this file will be converted to an image font_path - path to a font file (for example impact.ttf) """ grayscale = 'L' # parse the file into lines with open(text_path) as text_file: # can throw FileNotFoundError lines = tuple(l.rstrip() for l in text_file.readlines()) # choose a font (you can see more detail in my library on github) large_font = 20 # get better resolution with larger size font_path = font_path or 'cour.ttf' # Courier New. works in windows. linux may need more explicit path try: font = PIL.ImageFont.truetype(font_path, size=large_font) except IOError: font = PIL.ImageFont.load_default() print('Could not use chosen font. Using default.') # make the background image based on the combination of font and lines pt2px = lambda pt: int(round(pt * 96.0 / 72)) # convert points to pixels max_width_line = max(lines, key=lambda s: font.getsize(s)[0]) # max height is adjusted down because it's too large visually for spacing test_string = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ' max_height = pt2px(font.getsize(test_string)[1]) max_width = pt2px(font.getsize(max_width_line)[0]) height = max_height * len(lines) # perfect or a little oversized width = int(round(max_width + 40)) # a little oversized image = PIL.Image.new(grayscale, (width, height), color=PIXEL_OFF) draw = PIL.ImageDraw.Draw(image) # draw each line of text vertical_position = 5 horizontal_position = 5 line_spacing = int(round(max_height * 0.8)) # reduced spacing seems better for line in lines: draw.text((horizontal_position, vertical_position), line, fill=PIXEL_ON, font=font) vertical_position += line_spacing # crop the text c_box = PIL.ImageOps.invert(image).getbbox() image = image.crop(c_box) return image if __name__ == '__main__': main() -
howto: install python3 on fedora
https://developers.redhat.com/blog/2018/08/13/install-python3-rhel/
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Threads synchronization
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Locks have 2 states: locked and unlocked. 2 methods are used to manipulate them: acquire() and release()
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RLock is a reentrant lock. acquire() can be called multiple times by the same thread without blocking. Keep in mind that release() needs to be called the same number of times to unlock the resource.
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Condition is a synchronization mechanism where a thread waits for a specific condition and another thread signals that this condition has happened. Once the condition happened, the thread acquires the lock to get exclusive access to the shared resource.
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Semaphore is based on an internal counter which is decremented each time acquire() is called and incremented each time release() is called. If the counter is equal to 0 then acquire() blocks. It is the Python implementation of the Dijkstra semaphore concept: P() and V(). Using a semaphore makes sense when you want to control access to a resource with limited capacity like a server.
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Event a simple mechanism. A thread signals an event and the other thread(s) wait for it.
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Measure execution time (timeit)