Python 3.7: Cool New Features for You to Try

The breakpoint() Built-In

While we might strive to write perfect code, the simple truth is that we never do. Debugging is an important part of programming. Python 3.7 introduces the new built-in function breakpoint(). This does not really add any new functionality to Python, but it makes using debuggers more flexible and intuitive.

Assume that you have the following buggy code in the file bugs.py:

def divide(e, f):
    return f / e

a, b = 0, 1
print(divide(a, b))

Running the code causes a ZeroDivisionError inside the divide() function. Let’s say that you want to interrupt your code and drop into a debugger right at the top of divide(). You can do so by setting a so called “breakpoint” in your code:

def divide(e, f):
    # Insert breakpoint here
    return f / e

A breakpoint is a signal inside your code that execution should temporarily stop, so that you can look around at the current state of the program. How do you place the breakpoint? In Python 3.6 and below, you use this somewhat cryptic line:

def divide(e, f):
    import pdb; pdb.set_trace()
    return f / e

Here, pdb is the Python Debugger from the standard library. In Python 3.7, you can use the new breakpoint() function call as a shortcut instead:

def divide(e, f):
    breakpoint()
    return f / e

In the background, breakpoint() is first importing pdb and then calling pdb.set_trace() for you. The obvious benefits are that breakpoint() is easier to remember and that you only need to type 12 characters instead of 27. However, the real bonus of using breakpoint() is its customizability.

Run your bugs.py script with breakpoint():

$ python3.7 bugs.py 
> /home/gahjelle/bugs.py(3)divide()
-> return f / e
(Pdb)

The script will break when it reaches breakpoint() and drop you into a PDB debugging session. You can type c and hit Enter to continue the script. Refer to Nathan Jennings’ PDB guide if you want to learn more about PDB and debugging.

Now, say that you think you’ve fixed the bug. You would like to run the script again but without stopping in the debugger. You could, of course, comment out the breakpoint() line, but another option is to use the PYTHONBREAKPOINT environment variable. This variable controls the behavior of breakpoint(), and setting PYTHONBREAKPOINT=0 means that any call to breakpoint() is ignored:

$ PYTHONBREAKPOINT=0 python3.7 bugs.py
ZeroDivisionError: division by zero

Oops, it seems as if you haven’t fixed the bug after all…

Another option is to use PYTHONBREAKPOINT to specify a debugger other than PDB. For instance, to use PuDB (a visual debugger in the console) you can do:

$ PYTHONBREAKPOINT=pudb.set_trace python3.7 bugs.py

For this to work, you need to have pudb installed (pip install pudb). Python will take care of importing pudb for you though. This way you can also set your default debugger. Simply set the PYTHONBREAKPOINT environment variable to your preferred debugger. See this guide for instructions on how to set an environment variable on your system.

The new breakpoint() function does not only work with debuggers. One convenient option could be to simply start an interactive shell inside your code. For instance, to start an IPython session, you can use the following:

$ PYTHONBREAKPOINT=IPython.embed python3.7 bugs.py 
IPython 6.3.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: print(e / f)
0.0

You can also create your own function and have breakpoint() call that. The following code prints all variables in the local scope. Add it to a file called bp_utils.py:

from pprint import pprint
import sys

def print_locals():
    caller = sys._getframe(1)  # Caller is 1 frame up.
    pprint(caller.f_locals)

To use this function, set PYTHONBREAKPOINT as before, with the <module>.<function> notation:

$ PYTHONBREAKPOINT=bp_utils.print_locals python3.7 bugs.py 
{'e': 0, 'f': 1}
ZeroDivisionError: division by zero

Normally, breakpoint() will be used to call functions and methods that do not need arguments. However, it is possible to pass arguments as well. Change the line breakpoint() in bugs.py to:

breakpoint(e, f, end="<-END\n")

Run this code with breakpoint() masquerading as the print() function to see a simple example of the arguments being passed through:

$ PYTHONBREAKPOINT=print python3.7 bugs.py 
0 1<-END
ZeroDivisionError: division by zero

See PEP 553 as well as the documentation for breakpoint() and sys.breakpointhook() for more information.

Data Classes

The new dataclasses module makes it more convenient to write your own classes, as special methods like .__init__(), .__repr__(), and .__eq__() are added automatically. Using the @dataclass decorator, you can write something like:

from dataclasses import dataclass, field

@dataclass(order=True)
class Country:
    name: str
    population: int
    area: float = field(repr=False, compare=False)
    coastline: float = 0

    def beach_per_person(self):
        """Meters of coastline per person"""
        return (self.coastline * 1000) / self.population

These nine lines of code stand in for quite a bit of boilerplate code and best practices. Think about what it would take to implement Country as a regular class: the .__init__() method, a repr, six different comparison methods as well as the .beach_per_person() method. You can expand the box below to see an implementation of Country that is roughly equivalent to the data class:

class Country:

    def __init__(self, name, population, area, coastline=0):
        self.name = name
        self.population = population
        self.area = area
        self.coastline = coastline

    def __repr__(self):
        return (
            f"Country(name={self.name!r}, population={self.population!r},"
            f" coastline={self.coastline!r})"
        )

    def __eq__(self, other):
        if other.__class__ is self.__class__:
            return (
                (self.name, self.population, self.coastline)
                == (other.name, other.population, other.coastline)
            )
        return NotImplemented

    def __ne__(self, other):
        if other.__class__ is self.__class__:
            return (
                (self.name, self.population, self.coastline)
                != (other.name, other.population, other.coastline)
            )
        return NotImplemented

    def __lt__(self, other):
        if other.__class__ is self.__class__:
            return ((self.name, self.population, self.coastline) < (
                other.name, other.population, other.coastline
            ))
        return NotImplemented

    def __le__(self, other):
        if other