Python’Perl’vs.’Python’ • Perl is worse than Python because people wanted it worse. Larry...

Python

ESA 2011/2012 Adam Belloum

[email protected]

Material Prepared by Eelco Schatborn

ES: Python

•  Today: 1. Python introducCon 2. Basic Python: types, variables, statements, FuncCons 3. Regular expression 4. File and I/O 5. Modules 6. Packages 7. Object Oriented Perl 8. DocumentaCon

IntroducCon

•  WriUen by Guido van Rossum •  Started work in 1990 •  First release in 1991 •  Minor number release every 6 months •  Named aZer Monty Python

“Perl is executable line noise. Python is executable pseudo-‐code.”

What is Python

•  a general-‐purpose high-‐level programming language whose design philosophy emphasizes code readability.

•  Python aims to combine "remarkable power with very clear syntax”, and its standard library is large and comprehensive.

•  Its use of indentaCon for block delimiters is unusual among popular programming languages.

hUp://en.wikipedia.org/wiki/Python_%28programming_language%29

Perl vs. Python

•  Perl is worse than Python because people wanted it worse. Larry Wall, 14 Oct 1998

•  I would actively encourage my competition

to use Perl. Sean True, 30 Mar 1999

•  if you look at from 10 KM above, perl, python, and ruby are the same … guido van rossum , pycon-2012

Why I Love Python ©2001 www.BruceEckel.com

Last news about Python from Guido van Rossum

•  Keynote: Guido Van Rossum given at pycon-‐2012 (Same talk was given at the UvA last July) –  Trolls: a quesCon that isn’t meant as a quesCon, but to heckle •  “Python Sucks. Ruby Rules” •  “When will you admit Python 3 is a mistake?” •  “Since PyPy is so much faster than CPython, why not abandon CPython”

•  Etc.

Notes: hUps://andrew-‐schoen-‐pycon-‐2012-‐notes.readthedocs.org/en/latest/sunday/keynote.html

Video: hUp://ontwik.com/python/pycon-‐2012-‐keynote-‐guido-‐van-‐rossum/

10 Reasons to Learn the Python Programming Language

1.  Reduced CluUer 2.  It’s not backward-‐compa5ble in exchange for pain 3.  It doesn’t value performance over your producCvity 4.  It doesn’t treat you like stupid 5.  I don’t wait forever for a full implementaCon of the

language 6.  It doesn’t make assumpCons about how we discover

errors 7.  MarkeCng people are not involved 8.  You don’t have to type so much 9.  your guesses are usually right 10. Python lets you focus on concepts

Why I Love Python ©2001 www.BruceEckel.com

DocumentaCon : Pydoc

•  Commandline $ pydoc sys!!

•  Web browser: –  pydoc start an HTTP server on the local machine that will serve documentaCon to visiCng Web browsers. $ pydoc -p 1234 # start a HTTP server on port 1234!$ pydoc -g # start the server and ! # additionally bring up a small!!

Simple Python

A simple program to start with: #!/usr/bin/python principal = 1000 # initial amount rate = 0.05 # interest rate numyears = 5 # number of years year = 1 while year <= numyears: principal = principal*(1+rate) print year, principal year += 1

The Python Interpreter •  Normal method for scripCng using

#!/usr/bin/python

•  python is also an interacCve interpreter you can interpret single lines of code good for simple checking and debugging

$ python mypythonprogram.py!

$ python![GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin!

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

>>> !

ES: Python

•  Today: 1.  Python introducCon 2.  Basic Python: types, variables, statements, FuncCons 3.  Regular expression 4.  File and I/O 5.  Modules 6.  Packages 7.  Object Oriented Perl 8.  DocumentaCon

Variables

•  Python is a dynamically typed language: names can represent values of different types during the execuCon of the program.

•  Names or idenCfiers – must begin with a non-‐numeric character or underscore –  but may contain both numeric and non-‐numeric characters

Troll about dynamic type “I don’t want my app to fail witn an A9ributeError a;er running for 4 hours”

Dynamic types (Weak typing)

•  “You write what you want to do, let Python worry about how”

•  weakly typed programming languages are those that support implicit type conversion

•  Argument for –  dynamic typing is that it requires less effort on the part of the programmer, because the interpreter implicitly performs certain conversions

•  Argument against –  dynamic typing: “errors won’t be found”

Variables: Numbers (1) simple numbers the same as with perl: •  decimal: 12, -17, 255, … •  octal, start with 0: 015, -023, 0777, … •  hexadecimal, start with 0x: 0xc, -0x11, 0xff,…

floaCng point numbers: •  ”one and a quarter": 1.25 But also imaginary numbers: •  wriUen as 'numberj', for example: 1.23j

Variables: Numbers (2)

•  Complex numbers, using imaginary numbers: –  wriUen as: num1 + num2j, –  or can be created with: complex(real, imag) funcCon.

(1.0j * 1J) è (-1+0j)

Variables: Strings (1)

•  To create string literals, enclose them in single, double or triple quotes.

Examples: 'Hello World' "Python is groovy"

"""Sino si Pepito Biglangliko?"""

•  The same type of quote used to start the string must be used to terminate it.

Variables: String (2)

•  Escape sequences represent special characters: \n, \t, . . .

•  Both in double quoted (") and single quoted (') strings

•  For verba5m strings use raw strings using r'. . . ' >>> print "Hi there! \n \t It's me.\n" Hi there!

It's me.

>>> print r'And me \n \t as well!' And me \n \t as well!

Variables: Strings (3) •  Triple-‐quoted strings using either """or ‘’’ capture all the

text that appears before the terminaCng triple quote.

–  Single and double quoted strings must be on one logical line. –  Triple-‐quoted strings are useful when contents of the string span

mul5ple lines of text.

–  For example: >>> print """ … Usage: thingy [OPTIONS] … -h Display this usage message … -H hostname Hostname to connect to …"""

•  String literals can span mul5ple lines using a closing backslash ('\')

•  The backslash is for syntax purposes only, it is not included in the output

>>> print "Hi \ … there! \n\ … \t It's me.\n”

prints: Hi there! It's me.


Variables: Strings (5) •  Strings are indexed like lists, starCng at 0 •  You can use them using the index operator '[i]'

>>> a = "Hello World" >>> b = a[4] >>> b o

•  Substrings can be used by slicing: `[i:j]’ >>> a[0:6] "Hello " >>> d = a[7:] "World" >>> e = a[3:8] "lo Wo"


Other data types can be converted into a string using either str() or repr() funcCons or backquotes (`), which are a shortcut notaCon for repr().

Examples

>>> x = 5.2 >>> s = "The value of x is " + str(x) >>> s The value of x is 5.2

>>> s = "The value of y is " + repr(y) >>> s = "The value of y is " + `y`

Operators: numbers

•  The following operators for numbers apply: +, -, *, /, //, % …

•  C-‐style shiZing & masking 1<<16, x&0xff, x|1, ~x, x^y •  Integer division truncates

•  1/2 -‐> 0 # 1./2. -‐> 0.5, float(1)/2 -‐> 0.5 \\ >>> width = 20 >>> height = 5*9

>>> width * height 900

Operators: Strings

•  Strings can be concatenated using a '+' •  But also by wriCng them adjacent to each other:

Examples >>> print "Hello" + 'Python' HelloPython >>> print "Python " "says" ' Hello!'

Python says Hello!

Reference SemanCcs

•  Assignment manipulates references •  x = y does not make a copy of y •  x = y makes x reference the object y references

•  Very useful; but beware! •  Example:

>>> a = [1, 2, 3] >>> b = a >>> a.append(4) >>> print b [1, 2, 3, 4]

a

1 2 3

b

a

1 2 3

b

4

a = [1, 2, 3]

a.append(4)

b = a

a 1 2 3

Slide 25 ©2001, 2002 Guido van Rossum

a

1

b

a

1 b

a = 1

a = a+1

b = a

a 1

2

Changing an Integer

old reference deleted by assignment (a=...)

new int object created by add operator (1+1)

Slide 26 ©2001, 2002 Guido van Rossum

Play around 1

•  Write a short python prog, which when you enter our name it prints : Hi ! <your name> .

•  Hint: use raw_input() to get the input

>>> name = raw_input('What is your name?\n')

>>> What is your name? OS3

>>> print 'Hi, ' + name + '.' Hi, OS3.

Variables: Lists (1) •  lists and tuples are sequences of arbitrary objects You

can create a list as follows: >>> names = [ "Eric", "Trixie", "Coley" ]

They too are indexed: >>> a = names[2] a is now "Coley" >>> names[0] = ”Jan" >>> names[0] Jan

•  append a new member to the end of a list using append() >>> names.append("Khamir") >>> names [ "Eric", "Trixie", "Coley", "Khamir" ]

Variables: Lists (2) •  You can extract or reassign a por5on of a list by using the slicing

operator. Examples

>>> names = ["pusakat","Trixie","Coley","Khamir"] >>>names[0:2]

["pusakat", "Trixie”] >>> names[2:]

["Coley", "Khamir" ]

•  Use the plus ('+') operator to concatenate lists.

>>> a = [1,2,3] + [4,5] >>> a [1,2,3,4,5]

Variables: Lists (3) •  Lists can contain any kind of Python object including

other lists.

Example: >>> a = [1,"Dave”,3,["Mark”,9,[100, 101]],10]

•  Nested lists are accessed as follows: >>> a[1] "Dave" >>> a[3] ['Mark', 9, [100, 101]] >>> a[3][2][1] 101

Variables: Tuples

•  Tuples are a lot like lists –  Tuples support most of the same funcCons as a list –  They are however immutable aZer creaCon –  Used to return mul5ple values from a funcCon

•  You can create tuples by enclosing a group of values in parentheses ('(. . . )') or with a comma-‐separated list.

>>> a = (1,4,5,-9,10,'hello!’) >>> b = (7,) # this is a singleton >>> c = a, b >>> c ((1,4,5,-9,10,'hello!’),(7,))

Variables: DicConaries (1) •  A dic5onary is an associa5ve array or hash table that

contains objects indexed by keys. •  Only immutable objects can be used as a key, like

strings, numbers, tuples, etcetera. •  You create a dicConary by enclosing values in curly

braces ('{. . . }'): >>>> a = { … "username" : "xenos", … "home" : "/home/xenos", … "uid" :500 … } >>> a ["uid"] 500

Variables: DicConaries (2) •  Access any value using it's key:

>>> a["username"] "xenos" >>> a["home"] "/home/xenos”

•  To insert or modify objects, you assign a value to a key-‐indexed name.

a["username"] = "trixie" a["home"] = "/home/trixie" a["shell"] = "/usr/bin/tcsh”

Variables: DicConaries (3)

DicConary membership is tested with the has_key() method: if a.has_key("username"): username = a["username"]

else: username = "unknown user"

This can also be performed more compactly this way. username = a.get("username", "unknown user")

Variables: DicConaries (4)

•  To obtain a list of dicConary keys, use the keys() method.

k = a.keys() k = ["username", "home", "uid", "shell" ]

•  Use the del statement to remove an element of a dicConary.

del a["username"]

ES: Python

•  Today: 1.  Python introducCon 2.  Basic Python: types, variables, statements, funcCons 3.  Regular expression 4.  File and I/O 5.  Modules 6.  Packages 7.  Object Oriented Perl 8.  DocumentaCon

Flow control

•  A block of code contains a list of statements. •  Code blocks are denoted by using indenta5on •  Flow control can be exerted using looping or condiConal Statements.

Looping Statements (1)

•  Iterates over the members of a sequence, such as a string, list or tuple.

>>>for i in ( 1, 2, 3, 4, 5, 6, 7, 8, 9 ): … print "2 to the power %d is %d” % (i, 2**i)

•  Using the range() funcCon you can also give a range: >>> for i in range(1,10):

… print "2 to the power %d is %d” % (i, 2**i)


•  The for statement can iterate over any sequence type and isn't limited to sequences of integers. >>> a = "Hello World” # Print out the characters in a >>> for c in a:

… print c

>>> b = ["Eric", "Trixie", "Coley", "Khamir”]

# Print out the members of a list

>>> for name in b: … print name


# a = [0,1,2,3,4] # b = [1,2,3,4,5,6,7]

# c = [0,3,6,9,12] # d = [8,7,6,5,4,3,2]

•  The range(i, j) funcCon constructs a list of integers with values from i to j à 1.

•  If the starCng value is omiUed, it's assumed to be zero.

•  An opConal stride or step size can be given as a third •  argument.

a = range(5)

b = range(1,8) c = range(0,14,3)

d = range(8,1,-1)-1)

Play around 2

•  Write a short python prog, which iterate through a list and print the iteraCon

•  Hint: use enumerate() to iterate through the list

my_list = ['john', 'pat', 'gary', 'michael']

for i, name in enumerate(my_list):

print "iteration %i is %s" % (i, name))

Play Around 3

•  Write a short prog in python, which prints amount of money you have to pay for a given purchase let say: 1 kg (apples à1.40 euro/kg), 3 kg (banana à1.20 euro/kg)

•  Hint: use dicConary, prices = {'apple': 1.40, 'banana': 1.20} my_purchase = { 'apple': 1, 'banana': 6} grocery_bill = sum( prices[fruit] * my_purchase[fruit] \ for fruit in my_purchase) Print 'I owe the grocer $%.2f' % grocery_bill :

CondiConal Statements (1)

The if statement: if test: ... elif test: ... else:

...

•  The usual comparison operators for tesCng: <, >, ==, !=, <=, >=

•  They work on most Python objects

The while statement: while test:

...

CondiConal Statements (2) •  if statement example:

if a == 5: print "It's five!" elif a == 6: print "It's six!" else: print "It's something else.”

•  while statement example: a = 0 while a < 3: a = a +1 print "Counting up to 3..."

Play Around 4 •  Write a short prog in python which uses the local Cme to print

something ( your acCviCes during the day).

•  Hint: import Cme, Cme.localCme(), to get the hours myCme.tm_hour()

import time now = time.localtime() hour = now.tm_hour if hour < 8: print 'sleeping' elif hour < 9: print 'commuting' elif hour < 17: print 'working' elif hour < 18: print 'commuting' elif hour < 20: print 'eating' elif hour < 22: print 'resting' else: print 'sleeping'

ES: Python

•  Today: 1. Python introducCon 2. Basic Python: types, variables, statements, FuncCons 3. Regular expression 4. File and I/O 5. Packages 6. Object Oriented Perl 7. DocumentaCon

FuncCons (1)

•  funcCons can be defined using def def fibo(n): # calculate fibonacci up to n ...

•  arguments can have default values through assignment in the definiCon def fibo(n=100): # n has default value 100

...

FuncCons (2)

def name(arg1, arg2, ...): """documentaBon""" # opConal doc string statements return expression # from funcCon

FuncCon (3)

•  Return values without a return value a funcCon returns None

•  Example def fibo(n): # return Fibonacci series up to n result = []

a, b = 0, 1

while b < n: result.append(b) # see below

a, b = b, a+b

return resultturn result

FuncCon (4)

•  Usage –  calling the example funcCon without an argument >>> fibo() # call it [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

–  calling the example funcCon with an argument >>> fibo(50) # call it again [1, 1, 2, 3, 5, 8, 13, 21, 34]

Play Around 5

•  Write a short prog in python which has a funcCon with greets the name given as arguments

def greet(name): print 'hello', name

greet('Jack')

>>> hello, Jack

ES: Python


Regular Expressions

•  Import the re package to use regular expressions •  A number of funcCons are available in re package:

match()

search()

split() sub()

. . .

•  For more informaCon see the documentaCon $ pydoc re

Regular Expressions •  If we want to write a prog in python, which match a regexp to

validate a list of phone number in USA import re for test_string in ['555-1212', 'ILLEGAL’]:

if re.match(r'^\d{3}-\d{4}$', test_string): print test_string,'is a valid phone number'

else: print test_string, 'rejected'

Regular Expressions •  If we want to write a prog in python, which match a regexp to validate

a list of phone number in USA >>> import re >>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist") >>> m.group(0) # The enCre match 'Isaac Newton' >>> m.group(1) # The first parenthesized subgroup. 'Isaac' >>> m.group(2) # The second parenthesized subgroup. 'Newton' >>> m.group(1, 2) # MulCple arguments give us a tuple. ('Isaac’, ‘Newton’)

Play Around 5 •  Write a short prog in python, which open all files in you current directory

and prints its content

•  Hint: use glob module, –  open (filename) to open a file –  glob.glob('*.py’) Unix style pathname extensions – 

import glob # glob supports Unix style pathname extensions python_files = glob.glob('*.py') for fn in sorted(python_files):

print ' ------', fn for line in open(fn): print ' ' + line.rstrip()

print

Play Around 6 •  Write a short prog in python which sum up integers in

the command line •  Hint: important sys module, try: … except ValueError:

import sys

try: total = sum(int(arg) for arg in sys.argv[1:]) print 'sum =', total

except ValueError: print 'Please supply integer arguments'

ES: Python


Reading Files

name = open("filename") –  opens the given file for reading, and returns a file object

name.read() - file's entire contents as a string

>>> f = open("hours.txt") >>> f.read() '123 Susan 12.5 8.1 7.6 3.2\n 456 Brad 4.0 11.6 6.5 2.7 12\n 789 Jenn 8.0 8.0 8.0 8.0 7.5\n'

hUp://www.cs.washington.edu/educaCon/courses/cse143/12wi/python.shtml

Line-based File Processing

name.readline() - next line from file as a string

–  Returns an empty string if there are no more lines in the file

name.readlines() - file's contents as a list of lines

>>> f = open("hours.txt") >>> f.readline() '123 Susan 12.5 8.1 7.6 3.2\n'

>>> f = open("hours.txt") >>> f.readlines() ['123 Susan 12.5 8.1 7.6 3.2\n', '456 Brad 4.0 11.6 6.5 2.7 12\n', '789 Jenn 8.0 8.0 8.0 8.0 7.5\n']

Writing Files name = open("filename", "w") # write name = open("filename", "a") # append

–  opens file for write (deletes any previous contents) , or –  opens file for append (new data is placed after previous data)

name.write(str) - writes the given string to the file name.close() - closes file once writing is done

>>> out = open("output.txt", "w") >>> out.write("Hello, world!\n") >>> out.write("How are you?") >>> out.close()

>>> open("output.txt").read() 'Hello, world!\nHow are you?'


Line-based Input Template

•  A file object can be the target of a for ... in loop

•  A template for reading files in Python:

for line in open("filename"): statements

>>> for line in open("hours.txt"): ... print(line.strip()) # strip() removes \n 123 Susan 12.5 8.1 7.6 3.2 456 Brad 4.0 11.6 6.5 2.7 12 789 Jenn 8.0 8.0 8.0 8.0 7.5


ES: Python


Modules (1)

•  A module is a file containing Python defini5ons and statements.

•  The file name is the module name with the sux .py appended.

•  Within a module, the module's name (as a string) is available as the value of the global variable

__name__.

Modules (2)

•  Example module fibo.py

# Fibonacci numbers module

def fibo(n): # write Fibonacci # series up to n a, b = 0, 1

while b < n: print b a, b = b, a + b

Modules (3) •  Import the module with the following command:

>>> import fibo •  The funcCons are not included directly •  Using the module name you can access the funcCons:

>>> import fibo >>> fibo.fibo(1000) 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 >>> fibo.__name__ 'fibo'

Modules (4)

•  If you intend to use a funcCon oZen you can assign it to a local name:

>>> fib = fibo.fibo >>> fib(500) 1 1 2 3 5 8 13 21 34 55 89 144 233 377

Modules (5) •  The built-‐in funcCon dir() can be used to find out which

names a module defines. It returns a sorted list of strings: >>> import fibo >>> dir(fibo) ['__builtins__', '__doc__', '__file__', '__name__', '__package__', , 'fibo']

•  It lists all types of names: variables, modules, func5ons, etc.

•  Without arguments, dir() lists the names you have defined currently

ES: Python


Packages (1)

•  Packages are a way of structuring Python's module •  namespace by using doMed module names". •  They hide module names from other packages •  They are made up of modules in a parCcular hierarchic file system

•  Each directory name is apart of the module structure

Packages (2) •  An example Package:

Sound/ Top-level package __init__.py Initialization Formats/ file format conversions/ __init__.py wavread.py ... ...

•  __init__.py files are required to make Python treat the directories as containing package

•  __init__.py can just be an empty file, but it can also execute iniCalizaCon code for the package.

Packages (3) •  Import individual modules from the package:

import Sound.Effects.echo –  Any func5ons in a package must sCll be referenced by their fully qualified name Sound.Effects.echo.echofilter( ... )

•  An alternaCve way of imporCng the submodule is: from Sound.Effects import echo –  This also loads the submodule echo, and makes it available without its package prex

(Sound.Effects).

echo.echofilter( ... )

•  Import all modules from the package: from Sound.Effects import *

ES: Python


Class •  Declare a class by class <classname>:

–  class defini5ons introduce new a namespace, with its OWN scope –  all definiCons within a class definiCon are in that new scope

•  Use a class from <classname> import *

–  client programs must import the classes they use –  the file name (lowercase), not class name, is used

•  classes can be instan5ated into instance objects

from myclass import *!

x = MyClass() # It creates a new instance of the class and assigns this !! ! ! ! ! # object to the local variable x,!! ! ! ! ! # which now represents an instance object.!! ! ! ! ! !!! ! ! ! ! ! ! ! ! ! !

Class Method (Method Objects)

•  The first argument of any funcCon within a class is an object reference to a class instance

class MyClass: i = 12345 def f(self): return 'hello world’

•  A class constructor is a special method named def __init__(self [, param1, ..., param_n]): …

Class Method (Method Objects)

•  Class Methods –  The first argument is always the object itself def <methodname>(self, parm1, param2): self.param1 = … self.param2 = … …

•  Method objects are instan5a5ons of func5ons in a class.

–  the call method object through the instance object x.f()

Class instance (instance object) •  You can create/delete aMributes from an instance object (not

from a class object) –  create by assignment –  delete by using del.

x.counter = 1 # creates a new attribute of!! ! ! ! ! ! ! ! # the instance object x

while x.counter < 10:

x.counter *= 2 print x.counter

del x.counter # delete attribute from the !!! !! !! !! # instance object x

Example of class class BankAccount(object):

def __init__(self, initial_balance=0): self.balance = initial_balance

def deposit(self, amount): self.balance += amount

def withdraw(self, amount): self.balance -= amount

def overdrawn(self): return self.balance < 0

my_account = BankAccount(15) my_account.withdraw(5) print my_account.balance

hUp://wiki.python.org/moin/SimplePrograms

Printing Objects

•  By default, Python doesn't know how to print an object:

•  We'd like to be able to print a Point object and have its state shown as the output.

>>> p = Point(5, -2) >>> print p <Point instance at 0x00A8A850>


Printable Objects: __str__

def __str__(self): return string

–  converts an object into a string (like Java toString) –  invoked automatically when str or print is called def __str__(self): return "(" + str(self.x) + ", " + str(self.y) + ")"

>>> p = Point(5, -2) >>> print p (5, -2) >>> print "The point is " + str(p) + "!" The point is (5, -2)!


DocumentaCon

•  Use the web, there are a lot of websites on python •  Check www.python.org for help. •  Use the pydoc tool, python manual pages etcetera.

Material for these slides was taken from hUp://www.python.org/doc hUp://www.cs.washington.edu/educaCon/courses/cse143/12wi/python.shtml

Assignment •  Create a class python for keeping track of the various hardware

configuraCon and their relaCon to each other as used in our Lab. Note that there are different types of hardware and relaCons.

•  So, define a number of types of hardware and a number of types if relaCons (direct cable network, etc) and create a single class that can be instanCated to represent each and every one of them. The instance objects must be connected through relaCons to represent , in the end , the OS3 Lab in an abstract sense. So the objects represent a tree structure represenCng the lab as whole .

•  So as an example: your desktop machine has a keyboard a mouse and two screens directly aUached to it. The desktop computer itself is connected to the experimentaCon machines through a network. Then describe the experiment machines etc, etc.

Assignment •  The following funcCons must be made available for each node or piece of

hardware

•  Print –  should print the info of the certain piece only –  this includes the hardware descripCon at least. You could only also include it’s

posiCon Cn the lab/server room, its color etc. •  Printall

–  Should print he informaCon for this piece and all its descendant –  this should traverse the object downwards only

Bonus Thinks of way to create and a actual descripCon of the lab on top of the

abstract descripCon, so; add the name of a computer and who sits behind it etc. this implies that there can be many desktops computer instances that look exactly the same hardware wise, but have different owners.

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