If the replacements are going to be more in number, you can do this in this generic way:

strs, replacements = "abc&def#ghi", {"&": "\&", "#": "\#"}
 print "".join([replacements.get(c, c) for c in strs])
 # abc\&def\#ghi

#python #join #replace

To join lines in PyCharm first select you lines, then use CTRL+Shift+j (⌃⇧J) to put them in one line.

#pycharm #tricks #join #join_lines

What does select_related do in Django?

select_related does a join in case needed on the DB side and reduce query counts. Let's look at an example:

# Hits the database.
e = Entry.objects.get(id=5)

# Hits the database again to get the related Blog object.
b = e.blog

In the above code 2 queries are issued in DB side. First it gets Entry record and then blog is fetched from DB when e.blog is called. And here’s select_related lookup:

# Hits the database.
e = Entry.objects.select_related('blog').get(id=5)

# Doesn't hit the database, because e.blog has been prepopulated
# in the previous query.
b = e.blog

You can follow foreign keys in a similar way to querying them. If you have the following models:

from django.db import models

class City(models.Model):
    # ...
    pass

class Person(models.Model):
    # ...
    hometown = models.ForeignKey(
        City,
        on_delete=models.SET_NULL,
        blank=True,
        null=True,
    )

class Book(models.Model):
    # ...
    author = models.ForeignKey(Person, on_delete=models.CASCADE)

Then a call to Book.objects.select_related('author__hometown').get(id=4) will cache the related Person and the related City:

# Hits the database with joins to the author and hometown tables.
b = Book.objects.select_related('author__hometown').get(id=4)
p = b.author         # Doesn't hit the database.
c = p.hometown       # Doesn't hit the database.

# Without select_related()...
b = Book.objects.get(id=4)  # Hits the database.
p = b.author         # Hits the database.
c = p.hometown       # Hits the database.

#python #django #select_related #join #database #models