Lecture#2: Introductory Python

7/29/2019 Lecture#2: Introductory Python

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High performance computing

for non-programmers

Lecture #2:Python programminglanguage: Overview

Glib Ivashkevych

junior researcher, NSC KIPT


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cross-platform

simple to learn

general-purpose

flexible & extendable

Huge standard library

Tons of third-party packages

Documentation


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web-development

scripting in VFX

science & engineering

system administration

Youtube, Google, Instagram, DropBox

NASA, CERN

Philips, ILM and more

Is used for:


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data processing

glue layer

GUI

code generation & prototyping

Python has a lot of tools for scientific

computing: NumPy, SciPy, SymPy, scikit-learn

In computing:


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Scripting Compiled

language instructions areexecuted one by one

flexible, but slow

allow for very fastdevelopment cycle

perfect forcomplex automation

language instructions arecompiled to assembly

rigid, but fast

development cycle is usuallylonger

perfect forperformance


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How Python code is

executed

file.py file.pyc

sourcecode

bytecode

file.pyc

PythonVirtual

Machine

compilation

execution


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Python type system

Dynamic,

variables are references

no definitions, created automatically

but strong typing

objects have type

+ garbage collector


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x == y x is y

some string

x

y

True True


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x == y x is y

some string

x

y

True False

some string


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Mutable vs Immutable

Mutable objects can be changed

lists, dictionaries, sets, byte arrays

can be changed in-place

Immutable objects can not be changed once

creatednumbers, strings, tuples

new object is created at each

reassignment


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Naming

First symbol_ (underscore) or letter

Symbols_ (underscore), letters, digits

_myfunc

__somemagic

generate_file

generate-file

6func

fancy#name


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Numbers

Integer32 bit -2147483648...2147483647

long - arbitrary

boolean - True, False

Floatdouble precision, no support for single precision

Complextwo double precision numbers


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Sequences

List (mutable)

[1,2,Hello World, True]

Tuple (immutable)

(1,2,Hello World, True)

String (immutable)

HPC, new string, Some multiline string


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Mappings

Dictionary (mutable)

key-value storage

my_dict = {a:2, string:42, (1,2,3): [1,2,3]}

my_dict[a] == 2 #True

my_dict[(1,2,3)] == [1,2,3] #True

Only immutable types can be a key


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Logic

and, or, not

non-zero numbers, non-empty objects: True

zero-numbers, empty-objects, None: Falsecomparison and equality: True or False

and, or: return operand

if elif ... else statement

a ifb else c statement


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Blocks

Blocks are intended (with spaces, not tabs!)

ifx > y:

print x > y x = y

elifx == y: print X = Y

else: print X < Y

Empty block: pass


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Linebreaks

Break the line:

x = [1, 2, \3,4]

a = Somemultiline

string

x = [1, 2,3,4]

x = (1, 2,

3,4)

x = {a:1,

b:4)

x = {a:1,

b:4)


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Iterations

for i in some_iterable:

do_something(i)

some_iterable:

list, tuple, string, set, other iterable objects

while (x > 5):

do_something(x)

x += 1


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Functions

defmy_func(a, b=None):

print a and b are positional arguments

print b has default value

my_func(1) #the same as my_func(1, None)

my_func(1, 2)

Can have variable number of arguments


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Files

f = open(filename, mode), f.close()mode:

r - read (default),

w - write (existing file is erased),

a - append,

r+ - read and write

f.read() or f.read(size) #read entire file (be careful) or size bytes

f.readline() #read line

Iterate over file:

for line in f:

print line


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Questions?

Documents

Lecture#2: Introductory Python