Tag: パイソン

Errors sometimes occur when writing programs in Python.
Errors are actually helpful, since they tell you that your program has not been put together correctly,
but there are times when an error stops your program from running at all.

Sometimes code is syntactically correct yet still raises an error at runtime.
An error raised when the syntax is grammatically incorrect is called a syntax error,
while an error that is grammatically correct but logically wrong is called an exception.

Examples of exceptions include:
・TypeError: the operands have incompatible types
EX. word/2 , 4*number

・ZeroDivisionError: division by zero
EX. 3/0

・ValueError: the type is correct, but the value is not appropriate
EX. int(“string”)

You can prevent errors before they occur and write logically correct programs by using conditional branching, but another option is to handle errors with exception handling when they do occur.

That is where try, except comes in.

try: 
    実行したい処理(例外を含むかもしれない)
except エラー名:
    例外発生時に行う処理

You use it like this.
For example,

try:
    print(10 / 0)
except ZeroDivisionError:
    print('できませんでした')

#出力
できませんでした

This is the result.
Note, however, that except only catches the errors you specify
(in this program, only ZeroDivisionError), so any other error will still be reported as an error when the program runs.

When you expect more than one kind of error, add another “except ErrorName:” clause.

try:
    print(10 / 0)
except ZeroDivisionError:
    print('できませんでした')
except ValueError:
    print('値がうまく合致しませんでした')

You can specify multiple except clauses.

If you leave out the exception name in the except clause, it will catch every exception.

try:
    print(10 / 0)
except:
    print('できませんでした')

However, because this catches every exception, it will also hide errors that the programmer never anticipated, so use it with great care.

There are also related keywords in the try-except syntax that let you specify what happens after an exception occurs.
I will list them briefly here.
I plan to cover them in detail in another blog post.

• raise: deliberately raise an exception
• pass: do nothing after an exception occurs
• else: run only when no exception occurred
• finally: always run, whether or not an exception occurred

Arduino is normally controlled with the Arduino language.
Sooner or later, though, you will want to do something more complex, or control an Arduino from within another program.

The module introduced here, pySerial, lets you carry out serial communication with a Raspberry Pi or an Arduino.
Through serial communication, you can send commands from a Python program to an Arduino or Raspberry Pi and control them from Python.

Here I introduce the basic code for doing this.
Just being able to use this will greatly expand what your programs can do.

How to install

pip install pyserial

You can install it by opening Python in a terminal or command prompt.
Alternatively, you can install it from a terminal inside an IDE such as PyCharm.

Example code

This example is a program that turns an LED on and off at one-second intervals.
I will test it by writing output to pin 13, which drives the LED built into the Arduino.
On the Python side:

import serial, time

def main():
    #  COMポートを開く
    print("Open Port")
    ser = serial.Serial("COM3", 9600)
    while True:
        #  LED点灯
        ser.write(b"1")
        time.sleep(1)
        #  LED消灯
        ser.write(b"0")
        time.sleep(1)

    print("Close Port")
    ser.close()

if __name__ == '__main__':
    main()

serial.Serial specifies the port and the serial communication settings.
For the Arduino UNO, specify 9600.
This differs from board to board, so check it in the Arduino IDE.

The b in b”1″ plays a crucial role.
With the serial.write() function, numbers and strings must be converted to byte sequences before they can be sent over serial.
The b prefix is what marks the value as a byte sequence.

Since this program runs in an infinite loop, the LED keeps switching on and off every second until you stop the program.

Next is the program on the Arduino side.

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);
  digitalWrite(13, LOW);  //  初期化
}

void loop() {
  byte var;
  var = Serial.read();
  switch(var){
    case '0':
      digitalWrite(13, LOW);
      break;
    case '1':
      digitalWrite(13, HIGH);
      break;
    default:
      break;
  }
}

Here I use a switch-case statement.
It makes the program easier to follow.
For details, take a look at my previous blog post.

In this code, sending 0 sets the pin LOW and sending 1 sets it HIGH.
Using this program as a base, you can control an Arduino from a Python program in all sorts of ways.

When you start learning programming, I think the first hurdle is figuring out where to begin.
You search online, but then what? What software are you even supposed to use to write your code?

I struggled with exactly that myself.

Broadly speaking, there are two kinds of tools for writing programs.
One is interactive, and the other is script-based. (I’m putting it this way for clarity. I’m not entirely sure it’s the correct terminology, though…)

With the interactive type, you type one line and get one response back
—you write and run the program one line at a time, over and over.

With the script type, it’s like writing a whole essay and then getting feedback on it
—you run the entire program at once.
This script type is what most people picture when they think of programming: page after page of cryptic-looking code.

The problem is, when you set out to write a script-type program, what software should you actually use?

On top of that, beginner-level programming lessons often use the interactive style, but at the intermediate level you’re suddenly writing script code, and what to write it in is either left unstated or differs completely from book to book.

For the interactive style, you use something already built into your computer, like Command Prompt or Terminal, and get started by typing “Python”.

But what do you write script-type programs in?

Most script-type programs are written in an IDE (integrated development environment). (When in doubt, searching “Python IDE recommended” will turn up plenty of articles.)

The tools I normally use for writing script-type programs are
Jupyter Notebook and PyCharm.
(Apparently people are split on whether “Jupyter” is pronounced “joo-pih-ter” or “joo-py-ter”.)

I use Jupyter Notebook for statistical analysis and plotting graphs, and PyCharm for driving hardware or writing more complex programs.

Let me describe what each one is like.

First, Jupyter Notebook.
It comes bundled when you download Anaconda.
When you launch Jupyter Notebook, a browser such as Safari or Firefox opens first.

A list of the files on your computer is then displayed.
Open the folder you want, and click “New” in the upper right.
Then click “Python 3” and a screen like the image below will open.
Saving this file gives you a *.ipynb file.

You write your code in individual cells, and by clicking the RUN button at the top you can execute the program cell by cell.

Being able to run code one cell at a time is a huge advantage when making graphs or doing statistics.
That’s why I use it so often.

I use PyCharm when opening *.py files.
Note that the two tools open different file types, so be careful.

For PyCharm, please refer to other sites.
(I haven’t used it much yet, so I plan to cover how to use it in detail in a future post.)

The Anaconda site is here

The Jupyter Notebook site is here

The PyCharm site is here
For the download, choose the gray Community edition rather than Professional—that’s the free one.
I’d recommend starting with that.

I’ve been using Jupyter Notebook a lot lately, so I plan to keep posting updates, both as notes on my own learning and for anyone who wants to learn to program.