"Python Cookbook" "Data Structure and algorithm" 19. Simultaneous conversion and conversion of data

Problem: We need to invoke a conversion function (for example, sum (), Min (), Max ()), but first you need to convert or filter the data

Solution: Very elegant method---use builder expressions in function arguments

For example:

#calculate the sum of squaresnums=[1,2,3,4,5]s1=sum ((x*x forXinchnums)) S2=sum (x*x forXinchNums#more elegant usage.S3=sum ([x*x forXinchNums])#do not use builder expressionsPrint(S1)Print(S2)Print(S3)#determine if a. py file exists under a directoryImportOsfiles= Os.listdir (Os.path.expanduser ('~'))Print(Files)ifAny (Name.endswith ('. PY') forNameinchfiles):Print('there be python!')Else:    Print('Sorry, no python.')#Output a tuple as CSVs = ('ACME', 50, 123.45)Print(','. Join (STR (x) forXinchs))#data reduction across fields of a data structurePortfolio = [   {'name':'GOOG','shares': 50},   {'name':'YHOO','shares': 75},   {'name':'AOL','shares': 20},   {'name':'Scox','shares': 65}]min_shares= Min (s['shares'] forSinchportfolio)Print(min_shares) Min_shares2= Min (portfolio,key=Lambdas:s['shares'])#Using builder ExpressionsPrint(Min_shares2)

>>> ================================ RESTART ================================>>> 555555['. Idlerc','. Oracle_jre_usage','AppData','Application Data','Contacts','Cookies','Desktop','Documents','Downloads','Favorites','HelloWorld','Helloworld.zip','Links','Local Settings','log.html','Music','My Documents','MySite','Mysite.zip','NetHood','NTUSER. DAT','ntuser.dat.LOG1','ntuser.dat.LOG2','NTUSER. dat{6cced2f1-6e01-11de-8bed-001e0bcd1824}. TM.BLF','NTUSER. dat{6cced2f1-6e01-11de-8bed-001e0bcd1824}. Tmcontainer00000000000000000001.regtrans-ms','NTUSER. dat{6cced2f1-6e01-11de-8bed-001e0bcd1824}. Tmcontainer00000000000000000002.regtrans-ms','Ntuser.ini','Output.xml','Pictures','Pip','PrintHood','recent','report.html','Saved Games','Searches','SendTo','Templates','Videos','Start Menu']sorry, no python. ACME,50,123.4520{'shares': 20,'name':'AOL'}>>>

Summarize:

This scenario shows some of the grammatical subtleties of using a generator expression as a function parameter (that is, you do not have to reuse parentheses), for example, the following two lines of code represent the same meaning:

 for inch nums)) s for in nums)  # more Elegant usage
S3=sum ([x*x for X in Nums]) #不使用生成器表达式

It is more efficient and elegant to use generators to make parameters than to create a temporary list first.

"Python Cookbook" "Data Structure and algorithm" 19. Simultaneous conversion and conversion of data