Introduction to Python

Friday, 10 May 2019

Introduction to Python

Python versions

To find out which version the executable python refers to, execute this:

$ python --version

Python 2.7.15rc1

Dependencies & Package Management

Python Package Management with pip | My Public Notepad

Importing packages

import datetime
from jsonschema2db import JSONSchemaToPostgres

How do I import python file from multiple nested directories?
importing a module in nested packages

Getting Package Info

$ python3

Python 3.6.7 (default, Oct 22 2018, 11:32:17)

[GCC 8.2.0] on linux

Type "help", "copyright", "credits" or "license" for more information.

>>> import JSONSchema2DB

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

ModuleNotFoundError: No module named 'JSONSchema2DB'

>>> import jsonschema2db

>>> help(jsonschema2db)

>>> exit()

help(package_name) opens a documentation mode which lists all classes, functions etc.:

Help on module jsonschema2db:

NAME

jsonschema2db

CLASSES

builtins.object

JSONSchemaToDatabase

JSONSchemaToPostgres

JSONSchemaToRedshift

class JSONSchemaToDatabase(builtins.object)

| JSONSchemaToDatabase is the mother class for everything

| :param schema: The JSON schema, as a native Python dict

| :param database_flavor: Either "postgres" or "redshift"

...

FILE

/home/bojan/.local/lib/python3.6/site-packages/jsonschema2db.py

If we want to see docs for some particular class within the package we can start package examination from the top level and go deeper:

$ python3
Python 3.6.9 (default, Apr 18 2020, 01:56:04)
[GCC 8.4.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import scrapy
>>> help(scrapy)
>>> help(scrapy.http)
>>> help(scrapy.http.response)
>>> help(scrapy.http.response.Response)

Another example:

(venv) xxx:~/path/to/tensorflow-demo$ python
Python 3.6.8 (default, Oct 7 2019, 12:59:55)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> help
Type help() for interactive help, or help(object) for help about object.
>>> help("packages")
No Python documentation found for 'packages'.
Use help() to get the interactive help utility.
Use help(str) for help on the str class.

>>> help("modules")

...

_operator copyreg nis tensorflow

_osx_support crypt nntplib tensorflow_core

_pickle cryptography notebook tensorflow_estimator

...

>>> help("tensorflow")

>>> help("tensorflow_core")

pydoc is is Python module which generates documentation about modules, classes, functions and methods.

First run:

$ pydoc modules

$ python
Python 3.6.8 (default, Oct 7 2019, 12:59:55)
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> help('modules')

...to get a list of all modules.

Then run:

$ pydoc module

Example:

$ pydoc tensorflow_core

$ pydoc tensorflow_core.keras

$ pydoc tensorflow_core.keras.datasets.mnist

pydoc path:

$ which pydoc
/usr/bin/pydoc

pydoc can be used from virtual environment because. Virtual env's activate script exposes pydoc function (see section about virtual environments in this article).

python - Print everything that can be imported - Stack Overflow
terminal - In Python, what does pydoc do? - Stack Overflow

How to find package version programmatically?

Almost every normal package in python assigns the variable .__version__ or VERSION to the current version. So if you want to find the version of some package you can do the following

import a
a.__version__ # or a.VERSION

[source]

Functions

Example:

def add(a, b):

return a+b

Python 3 (3.5, from 2011) introduces (optional) function annotations:

def add(a: number, b: number) -> number:

return a+b

After importing some module, we can use dir(module) to list all its variables and functions. To find out types of arguments and return value of some function we can use inspect.signature or pydoc.render_doc. Example:

print("inspect.signature(mnist.load_data): {}".format(inspect.signature(mnist.load_data)))

print("pydoc.render_doc(mnist.load_data): {}".format(pydoc.render_doc(mnist.load_data)))

introspection - How do I look inside a Python object? - Stack Overflow
PEP 257 -- Docstring Conventions | Python.org

Classes

class MyClass:

In Python 3 each class implicitly inherit from object.

Inheritance

class Derived(Base):

Member Functions

class C:
def foo(self, a):
...

def bar(self, b):
...
self.foo(a)
...

Python call function within class - Stack Overflow

Data Types

List

Python List

Dictionary

Python Dictionary Methods
python dictionaries and JSON (crash course)

Environment

Find current directory and file's directory

Special Variables

__name__

__main__

python - What does if __name__ == "__main__": do? - Stack Overflow

When the Python interpeter reads a source file, it first defines a few special variables e.g. __name__ variable. If you are running your module (the source file) as the main program, e.g.

$ python foo.py

...the interpreter will assign the hard-coded string "__main__" to the __name__ variable.

If there's a statement like this in the main program, or in some other module the main program imports:

# Suppose this is in some other main program.

import foo

The interpreter will search for your foo.py file (along with searching for a few other variants), and prior to executing that module, it will assign the name "foo" from the import statement to the __name__ variable, i.e.

# It's as if the interpreter inserts this at the top

# of your module when it's imported from another module.

__name__ = "foo"