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Jeroen W. Pluimers on .NET, C#, Delphi, databases, and personal interests

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Archive for the ‘Python’ Category

PyGotham keynote: The Other Async (Threads + Async = ❤️)

Posted by jpluimers on 2019/09/18

Interesting talk:

Published on Oct 8, 2017

Screencast of my keynote presentation at PyGotham 2017, New York City. October 7, 2017. In this live-coded talk, I build a queue object that spans the world of threads and asyncio with a single unified API.

Via [WayBack] The Other Async (Threads + Async = ❤️) – screencast of David Beazley’s keynote at PyGotham 2017 – ThisIsWhyICode – Google+

–jeroen

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Posted in Development, Python, Scripting, Software Development | Leave a Comment »

What do the three arrow (“>>>”) signs mean in python?

Posted by jpluimers on 2019/09/10

When starting to work with Python, a lot of examples contain the >>> characters on the first line often followed by ... characters on continuing lines.

They are about two things:

  1. interactive Python sessions
  2. doctest

The answers in [WayBackWhat do the three arrow (“>>>”) signs mean in python? give insight in the various Python versions and how they prompt.

References from them:

–jeroen

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Python “NameError: name ‘socket’ is not defined”

Posted by jpluimers on 2019/09/05

I bumped into this a while ago, but could not find back the code example showing it, so below is the SO question to solve it:

NameError: name 'socket' is not defined

[WayBackHow to refer to a standard library in a logging configuration file?

Related: [WayBack[Tutor] Socket error in class

–jeroen

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python multithreading wait till all threads finished

Posted by jpluimers on 2019/09/04

A great tip from [WayBack] python multithreading wait till all threads finished:

ou need to use join method of Thread object in the end of the script.

t1 = Thread(target=call_script, args=(scriptA + argumentsA))
t2 = Thread(target=call_script, args=(scriptA + argumentsB))
t3 = Thread(target=call_script, args=(scriptA + argumentsC))

t1.start()
t2.start()
t3.start()

t1.join()
t2.join()
t3.join()

Thus the main thread will wait till t1t2 and t3 finish execution.

I’ve used a similar construct that’s used by the multi-threading code I posted a few ways ago (on Passing multiple parameters to a Python method: the * tag) in the ThreadManager class below.

But first some of the other links that helped me getting that code as it is now:

Example:

class ThreadManager:
    def __init__(self):
        self.threads = []

    def append(self, *threads):
        for thread in threads:
            self.threads.append(thread)

    def runAllToCompletion(self):
        ## The loops are the easiest way to run one methods on all entries in a list; see https://stackoverflow.com/questions/2682012/how-to-call-same-method-for-a-list-of-objects
        # First ensure everything runs in parallel:
        for thread in self.threads:
            thread.start()
        # Then wait until all monitoring work has finished:
        for thread in self.threads:
            thread.join()
        # here all threads have finished

def main():
    ## ...
    threadManager.append(
        UrlMonitorThread(monitor, "http://%s" % targetHost),
        SmtpMonitorThread(monitor, targetHost, 25),
        SmtpMonitorThread(monitor, targetHost, 587),
        SshMonitorThread(monitor, targetHost, 22),
        SshMonitorThread(monitor, targetHost, 10022),
        SshMonitorThread(monitor, targetHost, 20022))

    threadManager.runAllToCompletion()

–jeroen

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Python: variables in the class scope are class, not instance

Posted by jpluimers on 2019/09/03

A very subtle thing that keeps biting me as my background is from languages where by default, identifiers on the class scope are instance level, not class level:

In Python, variables on class level are class variables.

If you need instance variables, initialise them in your constructor with a self.variable = value.

The example in the Python 3 docs [WayBackClasses – A First Look at Classes – Class and Instance Variables is the same as in the Python 2 docs [WayBackClasses – A First Look at Classes – Class and Instance Variables:

Generally speaking, instance variables are for data unique to each instance and class variables are for attributes and methods shared by all instances of the class:

class Dog:

    kind = 'canine'         # class variable shared by all instances

    def __init__(self, name):
        self.name = name    # instance variable unique to each instance

>>> d = Dog('Fido')
>>> e = Dog('Buddy')
>>> d.kind                  # shared by all dogs
'canine'
>>> e.kind                  # shared by all dogs
'canine'
>>> d.name                  # unique to d
'Fido'
>>> e.name                  # unique to e
'Buddy'

For people new at Python: the __init__ is a constructor; see these links for more explanation:

Of course, the __init__() method may have arguments for greater flexibility. In that case, arguments given to the class instantiation operator are passed on to __init__(). For example,

>>> class Complex:
...     def __init__(self, realpart, imagpart):
...         self.r = realpart
...         self.i = imagpart
...
>>> x = Complex(3.0, -4.5)
>>> x.r, x.i
(3.0, -4.5)

–jeroen

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