Python 3 bare essentials

When you clone this repository a directory named pythrill will be created. The instructions and commands below assume that your working directory is the parent of pythrill. So, to run modtest.py you will do

python3 pythrill/modtest.py

Compile only

python3 -m py_compile script.py can be used to compile a script. A .pyc file is produced which can be ignored if you are just interested in syntax checking.

alias py3c="python3 -m py_compile"

Boolean

The bool class has only two possible instances True and False. Note the capitalisation.

I am having trouble explicitly passing False via argparse. Any string is interpreted as True, even the strings False and 0 (zero).

Constructors

There are constructors for the builtin types.

• int()
• float()
• complex()
• list()
• dict()
• set()

The argument to dict() is a list of tuples.

dict([(k1, v1), (k2,v2)])

Below also works. Think assignments to keys as arguments.

dict(k1 = v1, k2 = v2, k3 = v3)

The argument to set() is a list.

set([1,2,3])

Literal syntax

• int instantiated by context.
• float instantiated by context.
• complex instantiated by context.
• list made using [1, 2, 3, 4, "five", "six"]
• dict made using {k1:v1, k2:v2, k3:v3}
• tuple made using ("one", "two", 3, 4)
• set made using {"one", "two", 3, 4}

Note that empty braces {} are literal for an empty dictionary, not an empty set. To get an empty set use set().

pydoc3

In the documentation is function signatures the / (forward slash) denotes the end of positional only arguments. This can only be specified in the C API so you cannot do this when writing your own functions.

readinto(self, buffer, /)

Formatted output

The function print() is rather crude and useless.

for loop

for loops can have an else for them! Be careful!

Loop statements may have an else clause; it is executed when the loop terminates through exhaustion of the list (with for) or when the condition becomes false (with while), but not when the loop is terminated by a break statement. This is exemplified by the following loop, which searches for prime numbers:

for n in range(2, 10):
    for x in range(2, n):
        if n % x == 0:
            print(n, 'equals', x, '*', n//x)
            break
    else:
        # loop fell through without finding a factor
        print(n, 'is a prime number')

A module of commonly used functions

In the directory py3lib/ there is file named common.py. This is a collection of functions. They behave like some of those in my Sco::Common in Perl. Some may not be appropriate or needed in Python, and most can be written better.