读写文本文件

Question

I'm writing a project with the goal to build something like a managerial (business management, how to say), using Django.

Actually, i need the support of a global variable because products are identified by an increasing code, that sometime i reset ( it is NOT the primary key, just a code i use to work ). So, to avoid the usage of a global variable, also due to problem i would incour if the server reboots, i'd like to write the actual code on a text file.

With a save method overriding i keep the number written in this file and i use it to fill the product code field. Then i increase the number written in the file and close it. This way, i just need to write "0" (zero) with a text editor in the file to reset che code! And if the server reboots i don't lose the actual number like using other methods (ie: variables saved in cache)

The error i met is: no such file/directory called product_code.txt

Here the code of the model:

class Product(models.Model):

    code = models.AutoField(primary_key=True, editable=False)
    #category = models.ForeignKey(Category)

    product_code = models.IntegerField(editable=False, blank=True, null=True, verbose_name="codice prodotto")

    description = models.CharField(max_length=255, verbose_name="descrizione")
    agreed_price = models.DecimalField(max_digits=5, decimal_places=2, verbose_name="prezzo accordato")
    keeping_date = models.DateField(default=datetime.date.today(), verbose_name="data ritiro")

    selling_date = models.DateField(blank=True, null=True, verbose_name="data di vendita")
    discount = models.SmallIntegerField(max_length=2, default=0, verbose_name="sconto percentuale applicato")
    selling_price = models.DecimalField(
        max_digits=5, decimal_places=2, blank=True, null=True, verbose_name="prezzo finale"
    )
    sold = models.BooleanField(default=False, verbose_name="venduto")

    def save(self, force_insert=False, force_update=False, using=None, update_fields=None):

        if self.product_code is None:
            handle = open('product_code.txt', 'r+')
            actual_code = handle.read()
            self.product_code = int(actual_code)
            actual_code = int(actual_code) + 1  # Casting to integer to perform addition
            handle.write(str(actual_code))  # Casting to string to allow file writing
            handle.close()

        if self.sold is True:
            if self.selling_date is None or "":
                self.selling_date = datetime.date.today()
            if self.selling_price is None:
                self.selling_price = self.agreed_price - (self.agreed_price/100*self.discount)

        super(Product, self).save()

    class Meta:
        app_label = 'business_manager'
        verbose_name = "prodotto"
        verbose_name_plural = "prodotti"

The file product_code.txt is located in the same directory of models.py I ensure the error refer to the code line

handle = open('product_code.txt', 'r+')

since i checked it with the debugger.

Any idea to solve the problem? Thank you

Answer 1

You should open the file in append mode:

with open('product_code.txt', 'a') as handle:
    ...

But I would consider using another method (maybe a database) unless you know from the start that you are not going to see any race condition.