Confused: Newbie Function Calls
in [Python]
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From: fuglyducky on 11 Aug 2010 12:07 I am a complete newbie to Python (and programming in general) and I have no idea what I'm missing. Below is a script that I am trying to work with and I cannot get it to work. When I call the final print function, nothing prints. However, if I print within the individual functions, I get the appropriate printout. Am I missing something??? Thanks in advance!!!! ################################################ # Global variable sample_string = "" def gen_header(sample_string): HEADER = """ mymultilinestringhere """ sample_string += HEADER return sample_string def gen_nia(sample_string): NIA = """ anothermultilinestringhere """ sample_string += NIA return sample_string gen_header(sample_string) gen_nia(sample_string) print(sample_string)
From: Michael Torrie on 11 Aug 2010 12:30 On 08/11/2010 10:07 AM, fuglyducky wrote: > I am a complete newbie to Python (and programming in general) and I > have no idea what I'm missing. Below is a script that I am trying to > work with and I cannot get it to work. When I call the final print > function, nothing prints. However, if I print within the individual > functions, I get the appropriate printout. > > Am I missing something??? Thanks in advance!!!! Yes. You are passing sample_string into the functions, but not doing anything with the return value. > def gen_header(sample_string): ^^^^^^^^^^^^^^^ The sample_string name is rebound to the parameter now, completely hiding the global variable, if that's really what you wanted. > def gen_nia(sample_string): ^^^^^^^^^^^^^^^^^^^ Again. > NIA = """ > anothermultilinestringhere > """ > > sample_string += NIA > return sample_string > > > gen_header(sample_string) > gen_nia(sample_string) Try this: sample_string = gen_header(sample_string) sample_string = gen_nia(sample_string) You could drop the arguments to your functions and use the "global" keyword to get access to sample_string from within the functions, but normally that's a bad idea.
From: Pinku Surana on 11 Aug 2010 12:31 On Aug 11, 12:07 pm, fuglyducky <fuglydu...(a)gmail.com> wrote: > I am a complete newbie to Python (and programming in general) and I > have no idea what I'm missing. Below is a script that I am trying to > work with and I cannot get it to work. When I call the final print > function, nothing prints. However, if I print within the individual > functions, I get the appropriate printout. > > Am I missing something??? Thanks in advance!!!! > > ################################################ > # Global variable > sample_string = "" > > def gen_header(sample_string): > HEADER = """ > mymultilinestringhere > """ > > sample_string += HEADER > return sample_string > > def gen_nia(sample_string): > NIA = """ > anothermultilinestringhere > """ > > sample_string += NIA > return sample_string > > gen_header(sample_string) > gen_nia(sample_string) > > print(sample_string) There are 2 problems with your program. (1) If you want to use a global variable in a function, you have to add the line "global sample_string" to the beginning of that function. (2) Once you do (1), you will get an error because you've got sample_string as a global and a function parameter. Which one do you want to use in the function? You should change the name of the parameter to "sample" to solve that confusion. Here's the result, which works for me: sample_string = "" def gen_header(sample): global sample_string HEADER = """ mymultilinestringhere """ sample_string = sample + HEADER return sample_string def gen_nia(sample): global sample_string NIA = """ anothermultilinestringhere """ sample_string = sample + NIA return sample_string gen_header(sample_string) gen_nia(sample_string) print(sample_string)
From: Robert Kern on 11 Aug 2010 12:33 On 8/11/10 11:07 AM, fuglyducky wrote: > I am a complete newbie to Python (and programming in general) and I > have no idea what I'm missing. Below is a script that I am trying to > work with and I cannot get it to work. When I call the final print > function, nothing prints. However, if I print within the individual > functions, I get the appropriate printout. > > Am I missing something??? Thanks in advance!!!! > > ################################################ > # Global variable > sample_string = "" > > def gen_header(sample_string): > HEADER = """ > mymultilinestringhere > """ > > sample_string += HEADER > return sample_string By default, all assignments inside of a function are local to the function. Even augmented assignments like +=. Python strings are immutable, so sample_string += HEADER works exactly like sample_string = sample_string + HEADER The string referred to by the global name sample_string is never modified and the global name is never reassigned. http://docs.python.org/py3k/tutorial/classes.html#python-scopes-and-namespaces > gen_header(sample_string) > gen_nia(sample_string) > > print(sample_string) You probably want something like the following: sample_string = gen_header(sample_string) sample_string = gen_nia(sample_string) print(sample_string) -- Robert Kern "I have come to believe that the whole world is an enigma, a harmless enigma that is made terrible by our own mad attempt to interpret it as though it had an underlying truth." -- Umberto Eco
From: EW on 11 Aug 2010 12:39 This will work: ------------------------------------------------------------------------ sample_string="" def gen_header(sample_string=""): HEADER = """ mymultilinestringhere """ sample_string+= HEADER return sample_string def gen_nia(sample_string=""): NIA = """ anothermultilinestringhere """ sample_string += NIA return sample_string sample_string = gen_header(sample_string) sample_string = gen_nia(sample_string) print(sample_string) ------------------------------------------------------------------------ and this will work ------------------------------------------------------------------------ sample_string="" def gen_header(OtherString): global sample_string HEADER = """ mymultilinestringhere """ sample_string+= HEADER def gen_nia(OtherString): global sample_string NIA = """ anothermultilinestringhere """ sample_string += NIA gen_header(sample_string) gen_nia(sample_string) print(sample_string) ------------------------------------------------------------------------ The first one is the better of the 2 in this example but the second one will show you how to use global variables if you really need to use them So your problem was that you thought you were working on a global variable in your functions when you were not. Since the your def lines contained sample_string that make it a local variable. So when you were doing your += statements you were working on a local variable and not a global variable. You were returning the value of the local variable but you didn't have anything in the main body of your script catching that value. So simply changing these 2 lines: sample_string = gen_header(sample_string) sample_string = gen_nia(sample_string) made the global sample_string variable store the values of the return data. If you want to use global variables then you just have to do 2 things. First you have to make sure you don't have any local variables it the function with the same name. So I change the name to OtherString in the def line. Then you need a global statement at the start of your function (global sample_string) that tells python that you really do want to use that global variable. Global variables can cause you no end of heartache so python forces you to explicitly state that you want to use them. Hope that helps.
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