russellballestrini
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90 lines
3.3 KiB
ReStructuredText
90 lines
3.3 KiB
ReStructuredText
How to overload default function arguments in python using lambda
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:date: 2013-01-12 11:22
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:author: Russell Ballestrini
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:tags: Code, Greatest Hits, Guide
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:slug: how-to-overload-default-function-arguments-in-python-using-lambda
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:status: published
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Python Lambda functions are very powerful but I often forget how they
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work or the fun things they do. This post will document how to use a
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lambda to provide different default arguments to a function.
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We will use the `human function found in
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ago.py <https://bitbucket.org/russellballestrini/ago/overview>`__ as an
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example - because I'm the module author and I really like it. Lets use
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the interactive python interpreter to run help on the human function.
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::
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>>> from ago import human
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>>> help( human )
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Help on function human in module ago:
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human(dt, precision=2, past_tense='{} ago', future_tense='in {}')
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Accept a datetime or timedelta, return a human readable delta string
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.. raw:: html
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</p>
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Shown above the human function accepts 1 argument and 3 named keyword
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arguments. The dt argument must be a datetime or timedelta object, the
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precision must be an integer, and the other two must be strings. If we
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didn't like the default arguments, we would need to specify (or pass in)
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new values each time we invoked the function.
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Example: ``human(dt, 3, 'this happened {} ago!', 'in {} from now!')``.
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If we know we will always want different default arguments we can create
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a lambda function to shorten the invocation length.
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::
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>>> h = lambda dt : human(dt, 3, 'this happened {} ago!', 'in {} from now!')
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>>> print h( dt ) # h is much shorter then human, and still reusable!
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| Above creates a new function h who only accepts one argument dt. This
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function calls human with our default arguments.
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| This lambda is equivalent to this regular python function:
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::
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>>> def h( dt ):
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... return human(d, 3, 'this happened {} ago!', 'in {} from now!')
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>>> print h( dt ) # h is much shorter then human, and still reusable!
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Here is a working example to show the new lambda function h in action:
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::
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>>> from datetime import datetime
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>>> from datetime import timedelta
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>>> from ago import human
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>>>
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>>> h = lambda dt : human(dt, 3, 'this happened {} ago!', 'in {} from now!')
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>>>
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>>> present = datetime.now()
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>>>
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>>> for i in range( 1, 15 ):
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... if i % 2 == 0:
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... new_date = present - timedelta( i, i * i, i * i * i )
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... else:
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... new_date = present + timedelta( i, i * i, i * i * i )
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... h( new_date )
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...
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'in 22 hours, 9 minutes, 30 seconds from now!'
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'this happened 2 days, 1 hour, 50 minutes ago!'
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'in 2 days, 22 hours, 9 minutes from now!'
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'this happened 4 days, 1 hour, 50 minutes ago!'
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'in 4 days, 22 hours, 9 minutes from now!'
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'this happened 6 days, 1 hour, 51 minutes ago!'
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'in 6 days, 22 hours, 10 minutes from now!'
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'this happened 8 days, 1 hour, 51 minutes ago!'
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'in 8 days, 22 hours, 10 minutes from now!'
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'this happened 10 days, 1 hour, 52 minutes ago!'
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'in 10 days, 22 hours, 11 minutes from now!'
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'this happened 12 days, 1 hour, 52 minutes ago!'
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'in 12 days, 22 hours, 12 minutes from now!'
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'this happened 14 days, 1 hour, 53 minutes ago!'
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