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Basics

Python

Basics

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

A simple interpreted language

Python Basics


no separate compilation step

Python Basics

$$$$ python



$$$$ python

>>>>>>>>>>>>

Python Basics

$$$$ python



$$$$ python

>>>>>>>>>>>> print 1 + 2

3

>>>>>>>>>>>>

Python Basics

$$$$ python



$$$$ python

>>>>>>>>>>>> print 1 + 2

3

>>>>>>>>>>>> print 'charles' + 'darwin'

charlesdarwin

>>>>>>>>>>>>

Python Basics

Put commands in a file and execute that

Python Basics


$$$$ nano very-simple.py

Python Basics



print 1 + 2print 1 + 2

print 'charles' + 'darwin'

Python Basics



print 1 + 2print 1 + 2

print 'charles' + 'darwin'

$$$$ python very-simple.py

3

charlesdarwin

$$$$

Python Basics

Use an integrated development environment (IDE)

Python Basics


SourceSource

file

Python Basics


SourceSource

file

Python Basics

Execution

shell

Variables are names for values

Python Basics


Created by use

Python Basics


Created by use: no declaration necessary

Python Basics



>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>>

Python Basics



>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>> print planet

Pluto

>>>>>>>>>>>>

Python Basics



variablevariablevariablevariable valuevaluevaluevalue

planet 'Pluto'

>>>>>>>>>>>> planet = 'Pluto'


Pluto

>>>>>>>>>>>>

Python Basics



>>>>>>>>>>>> planet = 'Pluto'


Pluto

>>>>>>>>>>>> moon = 'Charon'

>>>>>>>>>>>>


planet

moon

'Pluto'

'Charon'

Python Basics



>>>>>>>>>>>> planet = 'Pluto'


Pluto

>>>>>>>>>>>> moon = 'Charon'

>>>>>>>>>>>> p = planet

>>>>>>>>>>>>


planet

moon

'Pluto'

'Charon'

Python Basics



>>>>>>>>>>>> planet = 'Pluto'


Pluto

>>>>>>>>>>>> moon = 'Charon'

>>>>>>>>>>>> p = planet

>>>>>>>>>>>>


planet

moon

p

'Pluto'

'Charon'

Python Basics



>>>>>>>>>>>> planet = 'Pluto'


Pluto

>>>>>>>>>>>> moon = 'Charon'

>>>>>>>>>>>> p = planet

>>>>>>>>>>>> print p

Pluto


planet

moon

p

'Pluto'

'Charon'

Python Basics

Pluto

>>>>>>>>>>>>

A variable is just a name

Python Basics


Does not have a type

Python Basics



>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>>

Python Basics



string>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>>planet 'Pluto'

variablevariablevariablevariable valuevaluevaluevaluestring

Python Basics



>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>> planet = 9


9


integer

Python Basics



>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>> planet = 9


9


Values are garbage collected

Python Basics




>>>>>>>>>>>> planet = 'Pluto'

>>>>>>>>>>>> planet = 9


9



Python Basics


If nothing refers to data any longer, it can be recycled

Must assign value to variable before using it

Python Basics


>>>>>>>>>>>> planet = 'Sedna'

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> planet = 'Sedna'

>>>>>>>>>>>> print plant # note the deliberate misspelling

Python Basics


>>>>>>>>>>>> planet = 'Sedna'


Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):

print plantprint plantprint plantprint plant

NameError: name 'plant' is not definedNameError: name 'plant' is not definedNameError: name 'plant' is not definedNameError: name 'plant' is not defined

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> planet = 'Sedna'


Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):Traceback (most recent call last):

Python does not assume default values for variables




>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> planet = 'Sedna'




Doing so can mask many errors




>>>>>>>>>>>>

Python Basics



>>>>>>>>>>>> planet = 'Sedna'








>>>>>>>>>>>>

Python Basics


Anything from # to the end of the line is a comment

Values do have types

Python Basics


>>>>>>>>>>>> string = "two"

>>>>>>>>>>>> number = 3

>>>>>>>>>>>> print string * number # repeated concatenation>>>>>>>>>>>> print string * number # repeated concatenation

twotwotwo

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> string = "two"

>>>>>>>>>>>> number = 3


twotwotwo

>>> >>> >>> >>> print string + number

Traceback (most recent call last)Traceback (most recent call last)Traceback (most recent call last)Traceback (most recent call last)

number + stringnumber + stringnumber + stringnumber + string

TypeError: cannot concatenate 'str' and 'int' objectsTypeError: cannot concatenate 'str' and 'int' objectsTypeError: cannot concatenate 'str' and 'int' objectsTypeError: cannot concatenate 'str' and 'int' objects

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> string = "two"

>>>>>>>>>>>> number = 3


twotwotwo





>>>>>>>>>>>>

Python Basics

Would probably be safe here to produce 'two3'


>>>>>>>>>>>> string = "two"

>>>>>>>>>>>> number = 3


twotwotwo





>>>>>>>>>>>>

Python Basics


But then what should '2'+'3' be?


>>>>>>>>>>>> string = "two"

>>>>>>>>>>>> number = 3


twotwotwo





>>>>>>>>>>>>

Python Basics


But then what should '2'+'3' be?

Doing too much is as bad as doing too little…

Use functions to convert between types

Python Basics


>>>>>>>>>>>> print int('2') + 3

5

>>>>>>>>>>>>>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> print int('2') + 3

5

>>> >>> >>> >>> print 2 + str(3)>>> >>> >>> >>> print 2 + str(3)

23

>>>>>>>>>>>>

Python Basics

Numbers

Python Basics

Numbers

14 32-bit integer

(on most machines)(on most machines)

Python Basics

Numbers

14 32-bit integer


14.0 64-bit float

(ditto)

Python Basics

Numbers

14 32-bit integer


14.0 64-bit float

(ditto)

1+4j complex number

(two 64-bit floats)

Python Basics

Numbers

14 32-bit integer


14.0 64-bit float

(ditto)

1+4j complex number

(two 64-bit floats)

Python Basics

x.real, x.imag real and imaginary parts of complex number

Arithmetic

Python Basics

Arithmetic

Addition + 35 + 22 57

Python Basics

Arithmetic

Addition + 35 + 22 57

'Py' + 'thon' 'Python''Py' + 'thon' 'Python'

Python Basics

Arithmetic

Addition + 35 + 22 57


Subtraction - 35 - 22 13

Python Basics

Arithmetic

Addition + 35 + 22 57



Multiplication * 3 * 2 6

Python Basics

Arithmetic

Addition + 35 + 22 57




'Py' * 2 'PyPy'

Python Basics

Arithmetic

Addition + 35 + 22 57




'Py' * 2 'PyPy'

Division / 3.0 / 2 1.5

Python Basics

Arithmetic

Addition + 35 + 22 57




'Py' * 2 'PyPy'

Division / 3.0 / 2 1.5

3 / 2 1

Python Basics

3 / 2 1

Arithmetic

Addition + 35 + 22 57




'Py' * 2 'PyPy'

Division / 3.0 / 2 1.5

3 / 2 1

Python Basics

3 / 2 1

Exponentiation ** 2 ** 0.5 1.41421356...

Arithmetic

Addition + 35 + 22 57




'Py' * 2 'PyPy'

Division / 3.0 / 2 1.5

3 / 2 1

Python Basics

3 / 2 1

Exponentiation ** 2 ** 0.5 1.41421356...

Remainder % 13 % 5 3

Prefer in-place forms of binary operators

Python Basics


>>>>>>>>>>>> years = 500

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1

>>>>>>>>>>>>>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1

>>>>>>>>>>>>

The same as years = years + 1>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1

>>> >>> >>> >>> print years>>> >>> >>> >>> print years

501

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1


501

>>>>>>>>>>>> years %= 10

>>>>>>>>>>>>

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1


501

>>>>>>>>>>>> years %= 10

>>>>>>>>>>>>

The same as years = years % 10

Python Basics


>>>>>>>>>>>> years = 500

>>> >>> >>> >>> years += 1


501

>>>>>>>>>>>> years %= 10

>>> >>> >>> >>> print years

5

>>>>>>>>>>>>

Python Basics

Comparisons

Python Basics

Comparisons

3 < 5 True

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 False

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 FalseSingle = is assignment

Double == is equality

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 False

3 >= 5 False

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 False

3 >= 5 False

1 < 3 < 5 True

Python Basics

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 False

3 >= 5 False

1 < 3 < 5 True

1 < 5 > 3 TrueBut please don't

do this

Python Basics

do this

Comparisons

3 < 5 True

3 != 5 True3 != 5 True

3 == 5 False

3 >= 5 False

1 < 3 < 5 True

1 < 5 > 3 True

3+2j < 5 errorerrorerrorerror

Python Basics

3+2j < 5 errorerrorerrorerror

October 2010

created by

Greg Wilson

October 2010

Copyright © Software Carpentry 2010

This work is licensed under the Creative Commons Attribution License

See http://software-carpentry.org/license.html for more information.

python - software carpentry · python basics. must assign value to variable before using it...

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