String & Representation

String & Representation

str()

  • print() -> str() -> __str(self)__
  • Fallback to repr(). By default, str() simply calls repr()
  • Produces a readable, human-friendly representation of an object
  • It is also the string constructor

repr()

  • Exactness is more important than human-friendliness
  • Suited for debugging. Unambiguous, precise, include type
  • Includes identifying information.
  • Generally best for logging and developers
  • The default repr() is not very helpful
  • As a rule, you should always write a repr() for your classes

  • standard library reprlib.repr() is a replacement for repr()

    • Example
    >>> l = ['a'] * 1000
>>> import reprlib
>>> reprlib.repr(l)
"['a', 'a', 'a', 'a', 'a', 'a', ...]"

format

  • "{:f}".format(obj) -> __format__(self, f)
  • Fallback to str()

built-in functions

  • ascii() replaces non-ASCII characters with escape sequences
  • chr() converts an integer Unicode codepoint to a single character string
  • ord() converts a single character to its integer Unicode codepoint

Numeric & scalar types

int

  • unlimited precision signed integer

  • bool in an int

    >>> False - True
-1
>>> False - False
0
>>> True - False
1
>>> True - True
0

float

  • IEEE-754 double precision (64-bit)
  • 53 bits of binary precision
  • 15 to 17 bits of decimal precision

  • Floating-point numbers are represented in computer hardware as base 2 (binary) fractions.

    • 0.001 has value 0/2 + 0/4 + 1⁄8. These two fractions have identical values, the only real difference being that the first is written in base 10 fractional notation, and the second in base 2

    • Unfortunately, most decimal fractions cannot be represented exactly as binary fractions. A consequence is that, in general, the decimal floating-point numbers you enter are only approximated by the binary floating-point numbers actually stored in the machine.

  • Example of float data

    
>>> f=0.9-0.8
>>> f
0.09999999999999998
>>> f=0.2-0.1
>>> f
0.1
>>> f=0.8-0.7
>>> f
0.10000000000000009
>>> float(2**53)
9007199254740992.0
>>> float(2**53+1)
9007199254740992.0
>>> float(2**53+2)
9007199254740994.0
>>> float(2**53+3)
9007199254740996.0
>>> float(2**53+4)
9007199254740996.0

decimal

  • standard library module decimal containing the class Decimal
  • decimal floating point configurable (although finite) precision defaults to 28 digits of decimal precision
  • identity is preserved. x == (x // y) * y + x % y, so integer division and modulus are consistent

  • Example

    >>> from decimal import Decimal
>>> Decimal(0.6)-Decimal(0.5)
Decimal('0.09999999999999997779553950750')
>>> Decimal('0.6')-Decimal('0.5')
Decimal('0.1')
>>> Decimal(0.2)-Decimal(0.1)
Decimal('0.1000000000000000055511151231')
>>> Decimal(0.8)-Decimal(0.7)
Decimal('0.1000000000000000888178419700')
>>> Decimal('0.8')-Decimal('0.7')
Decimal('0.1')

## Change the precise 
>>> import decimal
>>> decimal.getcontext().prec=4
>>> Decimal(-7) / Decimal(3)
Decimal('-2.333')

## Python handle different type in different way with % 
>>> Decimal(-7) % Decimal(3)
Decimal('-1')  ## 
>>> Decimal(-7) // Decimal(3)
Decimal('-2')  ## The next multiple of 3 towards zero is -6
>>> (-7) // (3) 
-3  ## The largest multiple of 3 less than -7 is -9
>>> (-7) % (3)
2  ## The

fraction

  • standard library module fractions containing the class Fraction for rational numbers

    • Denominator cannot be zero. e.g. 2 / 3, 2 is numerator, 3 is denominator.

  • Example

    
>>>from fractions import Fraction
>>>f = Fraction("2/3")
>>>f
Faction(2,3)
>>> Fraction(0.2)
Fraction(3602879701896397, 18014398509481984)
>>> Fraction(0.5)
Fraction(1, 2)
>>> Fraction(Decimal('0.3'))
Fraction(3, 10)
>>> Fraction(Decimal('0.3')) // Fraction(6, 7)
0
>>> Fraction(Decimal('0.3')) % Fraction(6, 7)
Fraction(3, 10)
>>> Fraction(Decimal('0.3')) - Fraction(6, 7)
Fraction(-39, 70)
>>> Fraction(Decimal('0.3')) + Fraction(6, 7)
Fraction(81, 70)
>>> Fraction(Decimal('0.3')) * Fraction(6, 7)
Fraction(9, 35)

>>> from math import floor, ceil
>>> ceil(Fraction('8/7'))
2
>>> floor(Fraction('8/7'))
1

number base conversions

bin() oct() hex() int(x, base)
base 2 base 8 base 16 bases 2 to 36
  • Example
>>> 0b0101
5
>>> 0o63527
26455
>>> 0o63
51
>>> 0xad2
2770
>>> hex(22)
'0x16'
>>> oct(22)
'0o26'
>>> bin(22)
'0b10110'

complex

  • complex construction string argument may have parentheses but must not contain spaces
  • cmath standard Library module contains complex equivalents of math

  • Example

    >>> 1+1j
(1+1j)
>>> type(1+1j)
<class 'complex'>
>>> (1+1j) + (1-1j)
(2+0j)
>>> (1+1j) + (2-2j)
(3-1j)
>>> (1+1j) - (2-2j)
(-1+3j)
>>> complex('-2+1j')
(-2+1j)
>>> complex('(-2+1j)')
(-2+1j)
>>> complex(-2, 1)
(-2+1j)

date & time

  • Gregorian calendar

  • weekday()

    0 Monday
1 Tuesday
2 Wednesday
3 Thursday
4 Friday
5 Saturday
6 Sunday

  • isoweekday()

    1 Monday
2 Tuesday
3 Wednesday
4 Thursday
5 Friday
6 Saturday
7 Sunday

  • timedelta

    • Constructor accepts and sums • days • seconds • microseconds • milliseconds • minutes • hours • weeks
    • Instances store only • days • seconds • microseconds

  • Example

    from datetime import (date, time)
from datetime import datetime as Datetime
from datetime import timedelta
from datetime import (tzinfo, timezone)

### date

>>> datetime.date(2000,month=2, day=10)
datetime.date(2000, 2, 10)
>>> datetime.date.today()
datetime.date(2014, 3, 14)
>>> datetime.date.fromtimestamp(99999999)
datetime.date(1973, 3, 3)
>>> datetime.date.fromordinal(9999)
datetime.date(28, 5, 17)
>>> datetime.date.max
datetime.date(9999, 12, 31)
>>> datetime.date.min
datetime.date(1, 1, 1)
>>> datetime.date.today().weekday()
4
>>> datetime.date.today().isoweekday()
5
>>> d = datetime.date.fromtimestamp(99999999)
>>> d.strftime('%A %d %B %b')
'Saturday 03 March Mar'
>>> d.strftime('%A %d %B %b %Y')
'Saturday 03 March Mar 1973'
>>> "The date is {:%A %d %B %b %y}".format(d)
'The date is Saturday 03 March Mar 73'
>>> "{date:%A} {date.day} {date:%B} {date:%Y}".format(date=d)
'Saturday 3 March 1973'

### time

>>> t=datetime.time(23,59,1,7451)   
>>> t.isoformat()
'23:59:01.007451'
>>> t.strftime('%Hh%Mm%Ss')
'23h59m01s'
>>> datetime.time.max
datetime.time(23, 59, 59, 999999)
>>> datetime.time.min
datetime.time(0, 0)

### datetime

>>> datetime.datetime(2001,6,7,8,15,25,895)
datetime.datetime(2001, 6, 7, 8, 15, 25, 895)
>>> dt= datetime.datetime(2001,6,7,8,15,25,895)
>>> dt.isoformat()
'2001-06-07T08:15:25.000895'

### timedelta
>>> td = datetime.timedelta(weeks=2, days=1, hours=1, minutes=1, microseconds=2, milliseconds= 1)
>>> td
datetime.timedelta(15, 3660, 1002)