{"id":2677,"date":"2016-06-07T08:45:49","date_gmt":"2016-06-07T16:45:49","guid":{"rendered":"http:\/\/blogs.msdn.microsoft.com\/pythonengineering\/?p=725"},"modified":"2019-02-17T15:27:08","modified_gmt":"2019-02-17T22:27:08","slug":"lambda-exp-unleashed-2","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/python\/lambda-exp-unleashed-2\/","title":{"rendered":"Python lambda expressions unleashed"},"content":{"rendered":"

Carl Kadie, Ph.D., is a research developer in Microsoft Research\/TnR working on Genomics.<\/em><\/p>\n

\"Launch<\/a><\/p>\n

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Lambda expressions<\/a> provide a way to pass functionality into a function. Sadly, Python puts two annoying restrictions on lambda expressions. First, lambdas can only contain an expression<\/a>, not statements<\/a>. Second, lambdas can’t be serialized to disk. This blog shows how we can work around these restrictions and unleash the full power of lambdas.<\/p>\n

So what are lambda’s good for? Suppose, you have a list of words from a string.<\/p>\n<\/div>\n<\/div>\n<\/div>\n

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In [1]:<\/div>\n
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<\/span>"This is a test string from Carl"<\/span>.<\/span>split<\/span>()<\/span>\n<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Out[1]:<\/div>\n
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['This', 'is', 'a', 'test', 'string', 'from', 'Carl']<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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You can sort the words with sorted()<\/code>.<\/p>\n<\/div>\n<\/div>\n<\/div>\n
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In [2]:<\/div>\n
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<\/span>sorted<\/span>(<\/span>"This is a test string from Carl"<\/span>.<\/span>split<\/span>())<\/span>\n<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Out[2]:<\/div>\n
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['Carl', 'This', 'a', 'from', 'is', 'string', 'test']<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Notice, however, that all the capitalized words, sort before all the lower-case words. This can be fixed by passing a lambda expression as the key<\/em> argument to the sorted()<\/code> function.<\/p>\n<\/div>\n<\/div>\n<\/div>\n
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In [3]:<\/div>\n
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<\/span>sorted<\/span>(<\/span>"This is a test string from Carl"<\/span>.<\/span>split<\/span>(),<\/span> key<\/span>=<\/span>lambda<\/span> word<\/span>:<\/span> word<\/span>.<\/span>lower<\/span>())<\/span>  # `key=str.lower` also works.<\/span>\n<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Out[3]:<\/div>\n
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['a', 'Carl', 'from', 'is', 'string', 'test', 'This']<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Lambda can be more complicated. Suppose we want to sort the words based on their (lower-case) back-to-front letters? As a reminder, here is a Python way to reverse the lower-case letters of a word:<\/p>\n<\/div>\n<\/div>\n<\/div>\n
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In [4]:<\/div>\n
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<\/span>str<\/span>.<\/span>lower<\/span>(<\/span>"Hello"<\/span>)[::<\/span>-<\/span>1<\/span>]<\/span>\n<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Out[4]:<\/div>\n
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'olleh'<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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And here is how to pass this functionality to sorted()<\/code> using a lambda:<\/p>\n<\/div>\n<\/div>\n<\/div>\n
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In [5]:<\/div>\n
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<\/span>sorted<\/span>(<\/span>"This is a test string from Carl"<\/span>.<\/span>split<\/span>(),<\/span> key<\/span>=<\/span>lambda<\/span> word<\/span>:<\/span> word<\/span>.<\/span>lower<\/span>()[::<\/span>-<\/span>1<\/span>])<\/span>\n<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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Out[5]:<\/div>\n
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['a', 'string', 'Carl', 'from', 'is', 'This', 'test']<\/pre>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n
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But what if you want even more complex functionality? For example, functionality that requires if<\/code> statements and multiple lines with unique scoping? Sadly, Python restricts lambdas to expressions only. But there is a workaround!<\/p>\n
Define a function that<\/p>\n