Adding a blog post explaining features introduced in Python 3.7

Author: ultimatecoder

_posts/2019-01-13-python-3-7-feature-walkthrough.md

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+---
+layout: post
+title: "Python 3.7 feature walkthrough"
+date: "2019-01-13 01:23:12 +0530"
+tag:
+  - Python
+  - CorePython
+  - Python3.7
+---
+
+
+In this post, I will explain improvements done in Core Python version 3.7. Below
+is the outline of features covered in this post.
+
+* Breakpoints
+
+* Subprocess
+
+* Dataclass
+
+* Namedtuples
+
+* Hash-based Python object file
+
+### breakpoint()
+
+Breakpoint is an extremely important tool for debugging. Since I started
+learning Python, I am using the same API for putting breakpoints. With this
+release, ```breakpoint()``` is introduced as a built-in function. Because it is
+in a built-in scope, you don't have to import it from any module. You can call
+this function to put breakpoints in your code. This approach is handier  than
+importing ```pdf.set_trace()```.
+
+![Breakpoint function in Python 3.7](/assets/images/walkthrough_python_3_7/breakpoint_example.gif)
+
+Code used in above example
+
+```python
+for i in range(100):
+    if i == 10:
+        breakpoint()
+    else:
+        print(i)
+
+```
+
+### PYTHONBREAKPOINT
+
+There wasn't any handy option to disable or enable existing breakpoints with a
+single flag. But with this release, you can certainly reduce your pain by using
+```PYTHONBREAKPOINT``` environment variable. You can disable all breakpoints in
+your code by setting the environment variable ```PYTHONBREAKPOINT``` to ```0```.
+
+
+![Breakpoint environment variable in Python 3.7](/assets/images/walkthrough_python_3_7/breakpoint_environment_variable_example.gif)
+
+##### I advise putting "PYTHONBREAKPOINT=0" in your production environment to avoid unwanted pausing at forgotten breakpoints
+
+
+### Subprocess.run(capture_output=True)
+
+You can pipe the output of Standard Output Stream (stdout) and Standard Error
+Stream (stderr) by enabling ```capture_output``` parameter of
+```subprocess.run()``` function.
+
+![subprocess.run got capture_output parameter](/assets/images/walkthrough_python_3_7/subprocess_run_capture_output.gif)
+
+You should note that it is an improvement over piping the stream manually. For
+example, ```subprocess.run(["ls", "-l", "/var"], stdout=subprocess.PIPE,
+stderr=subprocess.PIPE)``` was the previous approach to capture the output of
+```stdout``` and ```stderr```.
+
+
+### Dataclasses
+
+The new class level decorator ```@dataclass``` introduced with the
+```dataclasses``` module. Python is well-known for achieving more by writing
+less. It seems that this module will receive more updates in future which can be
+applied to reduce significant line of code. Basic understanding of Typehints is
+expected to understand this feature.
+
+When you wrap your class with the ```@dataclass``` decorator, the decorator will
+put obvious constructor code for you.  Additionally, it defines a behaviour for
+dander methods ```__repr__()```, ```__eq__()``` and ```__hash__()```.
+
+
+![Dataclasses.dataclass](/assets/images/walkthrough_python_3_7/dataclasses_dataclass.gif)
+
+Below is the code before introducing a ```dataclasses.dataclass``` decorator.
+
+```python
+class Point:
+
+    def __init__(self, x, y):
+        self.x = x
+        self.y = y
+```
+
+
+After wrapping with ```@dataclass``` decorator it reduces to below code
+
+```python
+from dataclasses import dataclass
+
+
+@dataclass
+class Point:
+    x: float
+    y: float
+```
+
+
+### Namedtuples
+
+The namedtuples are a very helpful data structure, yet I found it is less known
+amongst developers. With this release, you can set default values to argument
+variables.
+
+![Namedtuples with default arguments](/assets/images/walkthrough_python_3_7/namedtuple_example.gif)
+
+##### Note: Default arguments will be assigned from left to right. In the above example, default value ``2`` will be assigned to variable ``y``
+
+Below is the code used in the example
+
+```python
+from collections import namedtuple
+
+
+Point = namedtuple("Point", ["x", "y"], defaults=[2,])
+p = Point(1)
+print(p)
+```
+
+
+### .pyc
+
+**.pyc** are object files generated everytime you change your code file (.py).
+It is a collection of meta-data created by an interpreter for an executed code.
+The interpreter will use this data when you re-execute this code next time.
+Present approach to identify an outdated object file is done by comparing meta
+fields of source code file like last edited date. With this release, that
+identification process is improved by comparing files using a hash-based
+approach. The hash-based approach is quick and consistent across various
+platforms than comparing last edited dates. This improvement is considered
+unstable. Core python will continue with the metadata approach and slowly
+migrate to the hash-based approach.
+
+
+### Summary
+
+* Calling ```breakpoint()``` will put a breakpoint in your code.
+
+* Disable all breakpoints in your code by setting an environment variable
+  ```PYTHONBREAKPOINT=0```.
+
+* ```subprocess.run([...], capture_output=True)``` will capture the output of
+  ```stdout``` and ```stderr```.
+
+* Class level decorator ```@dataclass``` will define default logic for
+  constructor function. It will implement default logic for dunder methods
+  ```__repr__()```, ```___eq__()``` and ```__hash__()```.
+
+* Namedtuple data structure supports default values to its arguments using
+  ```defaults```.
+
+* Outdated Python object files (.pyc) are compared using the hash-based
+  approach.
+
+
+I hope you were able to learn something new by reading this post. If you want to
+read an in-depth discussion on each feature introduced in Python 3.7, then
+please read
+[this](https://docs.python.org/3.7/whatsnew/changelog.html#python-3-7-0-final)
+official post. Happy hacking!
+
+
+###### Proofreaders: [Jason Braganza](https://janusworx.com/), Ninpo, basen_ from #python at Freenode, Ultron from #python-offtopic at Freenode, up|ime from ##English at Freenode