# 4. Instances

## 4.1. Instance Attributes

bitmath objects have several instance attributes:

BitMathInstance.base

  The mathematical base of the unit of the instance (this will be 2 or 10)

      >>> b = bitmath.Byte(1337)
      >>> print b.base
      2

BitMathInstance.binary

  The Python binary representation of the instance’s value (in bits)

      >>> b = bitmath.Byte(1337)
      >>> print b.binary
      0b10100111001000

BitMathInstance.bin

  This is an alias for `binary`

BitMathInstance.bits

  The number of bits in the object

      >>> b = bitmath.Byte(1337)
      >>> print b.bits
      10696.0

BitMathInstance.bytes

  The number of bytes in the object

      >>> b = bitmath.Byte(1337)
      >>> print b.bytes
      1337

BitMathInstance.power

  The mathematical power the `base` of the unit of the instance is raised to

      >>> b = bitmath.Byte(1337)
      >>> print b.power
      0

BitMathInstance.system

  The system of units used to measure this instance (`NIST` or `SI`)

      >>> b = bitmath.Byte(1337)
      >>> print b.system
      NIST

BitMathInstance.value

  The value of the instance in prefix units1

      >>> b = bitmath.Byte(1337)
      >>> print b.value
      1337.0

BitMathInstance.unit

  The string representation of this prefix unit (such as `MiB` or `kb`)

      >>> b = bitmath.Byte(1337)
      >>> print b.unit
      Byte

BitMathInstance.unit_plural

  The pluralized string representation of this prefix unit.

      >>> b = bitmath.Byte(1337)
      >>> print b.unit_plural
      Bytes

BitMathInstance.unit_singular

  The singular string representation of this prefix unit (such as `MiB` or `kb`)

      >>> b = bitmath.Byte(1337)
      >>> print b.unit_singular
      Byte

Notes:

1. Given an instance `k`, where `k = KiB(1.3)`, then `k.value` is 1.3

The following is an example of how to access some of these attributes and what you can expect their printed representation to look like:

     1>>> dvd_capacity = GB(4.7)
     2>>> print "Capacity in bits: %s\nbytes: %s\n" % \
     3             (dvd_capacity.bits, dvd_capacity.bytes)
     4
     5Capacity in bits: 37600000000.0
     6bytes: 4700000000.0
     7
     8>>> dvd_capacity.value
     94.7
    10
    11>>> dvd_capacity.bin
    12'0b100011000001001000100111100000000000'
    13
    14>>> dvd_capacity.binary
    15'0b100011000001001000100111100000000000'

## 4.2. Instance Methods

bitmath objects come with a few basic methods: `to_THING()`, `format()`, and `best_prefix()`.

### 4.2.1. to_THING()

Like the available classes, there are 24 `to_THING()` methods available. `THING` is any of the bitmath classes. You can even `to_THING()` an instance into itself again:

     1>>> from bitmath import *
     2>>> one_mib = MiB(1)
     3>>> one_mib_in_kb = one_mib.to_kb()
     4>>> one_mib == one_mib_in_kb
     5True
     6
     7>>> another_mib = one_mib.to_MiB()
     8>>> print one_mib, one_mib_in_kb, another_mib
     91.0 MiB 8388.608 kb 1.0 MiB
    10
    11>>> six_TB = TB(6)
    12>>> six_TB_in_bits = six_TB.to_Bit()
    13>>> print six_TB, six_TB_in_bits
    146.0 TB 4.8e+13 Bit
    15
    16>>> six_TB == six_TB_in_bits
    17True

### 4.2.2. best_prefix()

best_prefix([system=None])

  Return an equivalent instance which uses the best human-readable prefix-unit to represent it.

  Parameters:

    system (int) – one of `bitmath.NIST` or `bitmath.SI`

  Returns:

    An equivalent `bitmath` instance

  Return type:

    `bitmath`

  Raises:

    ValueError – if an invalid unit system is given for `system`

The `best_prefix()` method returns the result of converting a bitmath instance into an equivalent instance using a prefix unit that better represents the original value. Another way to think of this is automatic discovery of the most sane, or human readable, unit to represent a given size. This functionality is especially important in the domain of interactive applications which need to report file sizes or transfer rates to users.

As an analog, consider you have 923,874,434¢ in your bank account. You probably wouldn’t want to read your bank statement and find your balance in pennies. Most likely, your bank statement would read a balance of $9,238,744.34. In this example, the input prefix is the cent: `¢`. The best prefix for this is the dollar: `$`.

Let’s, for example, say we are reporting a transfer rate in an interactive application. It’s important to present this information in an easily consumable format. The library we’re using to calculate the rate of transfer reports the rate in bytes per second from a `tx_rate()` function.

We’ll use this example twice. In the first occurrence, we will print out the transfer rate in a more easily digestible format than pure bytes/second. In the second occurrence we’ll take it a step further, and use the format method to make the output even easier to read.

    >>> for _rate in tx_rate():
    ...    print "Rate: %s/second" % Bit(_rate)
    ...    time.sleep(1)

    Rate: 100.0 Bit/sec
    Rate: 24000.0 Bit/sec
    Rate: 1024.0 Bit/sec
    Rate: 60151.0 Bit/sec
    Rate: 33.0 Bit/sec
    Rate: 9999.0 Bit/sec
    Rate: 9238742.0 Bit/sec
    Rate: 2.09895849555e+13 Bit/sec
    Rate: 934098021.0 Bit/sec
    Rate: 934894.0 Bit/sec

And now using a custom formatting definition:

    >>> for _rate in tx_rate():
    ...    print Bit(_rate).best_prefix().format("Rate: {value:.3f} {unit}/sec")
    ...    time.sleep(1)

    Rate: 12.500 Byte/sec
    Rate: 2.930 KiB/sec
    Rate: 128.000 Byte/sec
    Rate: 7.343 KiB/sec
    Rate: 4.125 Byte/sec
    Rate: 1.221 KiB/sec
    Rate: 1.101 MiB/sec
    Rate: 2.386 TiB/sec
    Rate: 111.353 MiB/sec
    Rate: 114.123 KiB/sec

### 4.2.3. format()

BitMathInstance.format(fmt_spec)

  Return a custom-formatted string to represent this instance.

  Parameters:

    fmt_spec (str) – A valid formatting mini-language string

  Returns:

    The custom formatted representation

  Return type:

    `string`

bitmath instances come with a verbose built-in string representation:

    >>> leet_bits = Bit(1337)
    >>> print leet_bits
    1337.0 Bit

However, for instances which aren’t whole numbers (as in `MiB(1/3.0) == 0.333333333333 MiB`, etc), their representation can be undesirable.

The `format()` method gives you complete control over the instance’s representation. All of the instances attributes are available to use when choosing a representation.

The following sections describe some common use cases of the `format()` method as well as provide a brief tutorial of the greater Python formatting meta-language.

#### 4.2.3.1. Setting Decimal Precision

By default, bitmath instances will print to a fairly long precision for values which are not whole multiples of their prefix unit. In most use cases, simply printing out the first 2 or 3 digits of precision is acceptable.

The following examples will show us how to print out a bitmath instance in a more human readable way, by limiting the decimal precision to 2 digits.

First, for reference, the default formatting:

    >>> ugly_number = MB(50).to_MiB() / 8.0
    >>> print ugly_number
    5.96046447754 MiB

Now, let’s use the `format()` method to limit that to two digits of precision:

    >>> print ugly_number.format("{value:.2f}{unit}")
    5.96 MiB

By changing the 2 character, you increase or decrease the precision. Set it to 0 (`{value:.0f}`) and you have what effectively looks like an integer.

#### 4.2.3.2. Format All the Instance Attributes

The following example prints out every instance attribute. Take note of how an attribute may be referenced multiple times.

     1>>> longer_format = """Formatting attributes for %s
     2   ...: This instances prefix unit is {unit}, which is a {system} type unit
     3   ...: The unit value is {value}
     4   ...: This value can be truncated to just 1 digit of precision: {value:.1f}
     5   ...: In binary this looks like: {binary}
     6   ...: The prefix unit is derived from a base of {base}
     7   ...: Which is raised to the power {power}
     8   ...: There are {bytes} bytes in this instance
     9   ...: The instance is {bits} bits large
    10   ...: bytes/bits without trailing decimals: {bytes:.0f}/{bits:.0f}""" % str(ugly_number)
    11
    12>>> print ugly_number.format(longer_format)
    13Formatting attributes for 5.96046447754 MiB
    14This instances prefix unit is MiB, which is a NIST type unit
    15The unit value is 5.96046447754
    16This value can be truncated to just 1 digit of precision: 6.0
    17In binary this looks like: 0b10111110101111000010000000
    18The prefix unit is derived from a base of 2
    19Which is raised to the power 20
    20There are 6250000.0 bytes in this instance
    21The instance is 50000000.0 bits large
    22bytes/bits without trailing decimals: 6250000/50000000

Note

On line 4 we print with 1 digit of precision, on line 16 we see the value has been rounded to 6.0

## 4.3. Instance Properties

### 4.3.1. THING Properties

Like the available classes, there are 24 `THING` properties available. `THING` is any of the bitmath classes. Under the covers these properties call `to_THING`.

     1>>> from bitmath import *
     2>>> one_mib = MiB(1)
     3>>> one_mib == one_mib.kb
     4True
     5
     6>>> print one_mib, one_mib.kb, one_mib.MiB
     71.0 MiB 8388.608 kb 1.0 MiB
     8
     9>>> six_TB = TB(6)
    10>>> print six_TB, six_TB.Bit
    116.0 TB 4.8e+13 Bit
    12
    13>>> six_TB == six_TB.Bit
    14True

## 4.4. The Formatting Mini-Language

That is all you begin printing numbers with custom precision. If you want to learn a little bit more about using the formatting mini-language, read on.

You may be asking yourself where these `{value:.2f}` and `{unit}` strings came from. These are part of the Format Specification Mini-Language which is part of the Python standard library. To be explicitly clear about what’s going on here, let’s break the first specifier (`{value:.2f}`) down into it’s component parts:

    {value:.2f}
       |  |||
       |  |||\---- The "f" says to format this as a floating point type
       |  ||\----- The 2 indicates we want 2 digits of precision (default is 6)
       |  |\------ The '.' character must precede the precision specifier for floats
       |  \------- The : separates the attribute name from the formatting specification
       \---------- The name of the attribute to print

The second specifier (`{unit}`) says to format the `unit` attribute as a string (string is the default type when no type is given).

See also

Python String Format Cookbook

  Marcus Kazmierczak’s excellent introduction to string formatting