Summer of code 2017: Python, Day 2

As explained in my Summer of code 2017: Python post I decided to pick up Python

Today is officially day 2

I decided to do a lunch and learn today and continue with the PluralSight course: Python Fundamentals

The course continued with Strings. Strings are very similar as strings in other languages

Here are some of the notes for myself of what I learned today


You can use single or double quotes with strings

Universal Newlines, you can use "\n" and it will be a newline on all platforms, this is also part of PEP 278

Multi-line strings are created by using triple quotes

For example

>>> x = """fdfdfdf
dfd
fdf
df
dfdf"""
>>> x
'fdfdfdf\ndfd\nfdf\ndf\ndfdf'
>>> 

As you can see, when the console prints the string, you will see the \n escape characters

Here are some of these escape characters

Escape Sequence	Meaning	Notes
\newline	Backslash and newline ignored
\\	Backslash (\)
\'	Single quote (')
\"	Double quote (")
\a	ASCII Bell (BEL)
\b	ASCII Backspace (BS)
\f	ASCII Formfeed (FF)
\n	ASCII Linefeed (LF)
\r	ASCII Carriage Return (CR)
\t	ASCII Horizontal Tab (TAB)
\v	ASCII Vertical Tab (VT)
\ooo	Character with octal value ooo
\xhh	Character with hex value hh

Escape sequences only recognized in string literals are:

Escape Sequence	Meaning	Notes
\N{name}	Character named name in the Unicode database
\uxxxx	Character with 16-bit hex value xxxx
\Uxxxxxxxx	Character with 32-bit hex value xxxxxxxx

More about that here: https://docs.python.org/3/reference/lexical_analysis.html#strings

Formatted String Literals

A formatted string literal or f-string is a string literal that is prefixed with 'f' or 'F'. These strings may contain replacement fields, which are expressions delimited by curly braces {}. While other string literals always have a constant value, formatted strings are really expressions evaluated at run time.

Here is an example

>>> name = "Denis"
>>> f"He said his name is {name!r}."
"He said his name is 'Denis'."
>>> 

Strings are unicode and encoded in UTF-8

After strings, the course continued with bytes, lists, dictionaries and loops

Modularity

This was the next part of this course
Named functions are defned with the def keyword

def function_name(arg1, argn):


For example

>>> def sumvalues(x,y):
    return x + y

>>> sumvalues(4,5)
9
>>> 

If you use a return without a value, the function will return None, but remember that None is not printed from the REPL

If you were to save this function into a file and then import it, it would execute immediately

Python has special attributes that start and end with double underscores, we will look at __main__ and  __name__

'__main__' is the name of the scope in which top-level code executes. A module’s __name__ is set equal to '__main__' when read from standard input, a script, or from an interactive prompt.

A module can discover whether or not it is running in the main scope by checking its own __name__, which allows a common idiom for conditionally executing code in a module when it is run as a script or with python -m but not when it is imported:

Here is an example of that

>>> 
def sumvalues(x,y):
    return x + y

    if __name__ == "__main__":
        sumvalues(x,y)

>>> sumvalues(1,2)
3
>>> 

More here: https://docs.python.org/3/library/__main__.html


I also learned about setting up a main() function

Command line arguments are accessible via sys.argv

This module also covered documenting your code by using docstrings. 

Docstrings are standalone literal strings which are the fist statement
of a function or module

If you have this function

>>> def fib2(n):  # return Fibonacci series up to n
    """Return a list containing the Fibonacci series up to n."""
    result = []
    a, b = 0, 1
    while a < n:
        result.append(a)    # see below
        a, b = b, a+b
    return result

Now if you do a help(fib2) command, you will see the following

>>> help(fib2)
Help on function fib2 in module __main__:

fib2(n)
    Return a list containing the Fibonacci series up to n.

>>> 

I feel I need to spend more time with __main__ and __name__.  

I will do that the next couple of days