[Maya-Python] Parsing flags using OR operator

[Marcus Ottosson]

Hi all,

What's a Pythonic way of parsing a set of flags, similar to how Qt does it.

E.g. layout.setAlignment(Qt.AlignTop | Qt.AlignLeft)

Both AlignTop and AlignLeft seem to be hex numbers (docs here) that seem to get combined into a third number via the | operator. Where's the logic behind that and how can I derive such logic myself?

Thanks,

Marcus

--

Marcus Ottosson
konstr...@gmail.com

[Justin Israel]

They are bitwise flags. Each value in a given enum is incremented by shifting bits. Then you are able to use bitwise operators to combine multiple values and test their membership:

https://wiki.python.org/moin/BitwiseOperators

A = 1 << 0

B = 1 << 1

C = 1 << 2

D = 1 << 3

print A,B,C,D

# 1 2 4 8

print A | B

# 3

print A | B | B | B

# 3

flag = A|B

if flag & A: print True

# True

if flag & B: print True

# True

if flag & C: print True

#

flag ^ B    # toggle off B

# 5

flag ^ B ^ B  # toggle off and on B

# 7

flag & ~B  # subtract B

# 5

flag & ~B & ~B  # maintains the subtraction (not toggle)

# 5

flag &= ~B  # unary subtraction; assignment

# 5

Basically, once you define a set of flags, you have AND, OR, XOR, NOT operators to test and modify the flags. It lets you represent a number of states in a single value.

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[Marcus Ottosson]

Brilliant, that makes sense.

I also came across this thread, the accepted answer is especially helpful.

http://stackoverflow.com/questions/276706/what-are-bitwise-operators

Thanks, Justin

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[Marcus Ottosson]

What do you think my usage of flags within this "policy" for a os.listdir-type object?

class ExcludePolicy(object):

# Flags

ExcludeHidden = 1 << 0

ExcludeInvisible = 1 << 1

ExcludeRegex = 1 << 2

ExcludeFiles = 1 << 3

def __init__(self):

self.__flags = self.ExcludeHidden | self.ExcludeInvisible | self.ExcludeFiles

self.__regex = ''

@property

def regex(self):

return self.__regex

@regex.setter

def regex(self, regex):

self.__regex = regex

@property

def flags(self):

return self.__flags

@flags.setter

def flags(self, flags):

self.__flags = flags

class ListDir(object):

def __init__(self):

self.__excludePolicy = None

def dir(self):

if not self.__excludePolicy:

return

pol = self.__excludePolicy

if pol.flags & pol.ExcludeHidden:

print "Excluding hidden"

if pol.flags & pol.ExcludeInvisible:

print "Excluding invisible"

if pol.flags & pol.ExcludeRegex:

print 'Excluding regex: "%s"' % pol.regex

if pol.flags & pol.ExcludeFiles:

print "Excluding files"

@property

def excludePolicy(self):

return self.__excludePolicy

@excludePolicy.setter

def excludePolicy(self, policy):

self.__excludePolicy = policy

if __name__ == '__main__':

pol = ExcludePolicy()

# pol.regex = r'^*.$'

# pol.flags = pol.ExcludeRegex

ld = ListDir()

ld.excludePolicy = pol

ld.dir()

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Marcus Ottosson
konstr...@gmail.com

[Justin Israel]

That looks right to me!

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[Marcus Ottosson]

Woho!

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Marcus Ottosson
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