Pythonic Programming by Dmitry Zinoviev

Author:Dmitry Zinoviev [Dmitry Zinoviev]
Language: eng
Format: epub
Publisher: Pragmatic Bookshelf
Published: 2021-09-23T00:00:00+00:00

Tip 52 Let the Caller Print

★★2.7, 3.4+ Functions or methods are usually considered as units of computation. They take the arguments (or rely on global variables, see Tip 92, ​Remember, There Are No Globals​), apply your algorithms to them, compute the results, and return them to the caller. If a function prints the calculated result instead of returning it, the caller cannot use the result for further computations. Moreover, the caller may be duped to believe that what the function returns is the result (see Tip 50, ​Make Functions Always Return Something​), but it is not. Here is a wrong way to produce a result:

​ ​def​ ​add1​(x):

​ ​print​(x+1)

​ ​# There is an implicit return None on this line!​

​ y = add1(10)

​=> ​11​

​ ​print​(y)

​=> ​None​

Here is a right way:

​ ​def​ ​add1​(x):

​ ​return​ x+1

​ y = add1(10)

​ ​print​(y)

​=> ​11​

Aside from the right and wrong ways, there is also a questionable way when a function prints the result and then returns it:

​ ​def​ ​add1​(x):

​ ​print​(x+1)

​ ​return​ x+1

​ y = add1(10)

​=> ​11​

​ ​print​(y)

​=> ​11​

This function, while functionally correct, combines computation and presentation. It always displays the result (and perhaps some other messages) and returns the result to the caller. The caller has no control over the function’s printout. Since printing is slow (see Tip 63, ​Build, Then Print​), if you call a “talkative” function in a loop, the performance of your code may significantly degrade. Also, it may be hard to see essential results buried in the sea of chatter. Let the caller of the function decide whether the returned value is worth printing.

You may still want to have an option of printing the result before returning it (say, for a very legitimate purpose of debugging). Do so by making printing optional and controllable by the caller, as explained in Tip 55, ​Pass Arguments Your Way​:

​ ​def​ ​add1​(x, debug=None):

​ ​if​ debug:

​ ​print​(x+1)

​ ​return​ x+1

​ y = add1(10)

​ y = add1(10, True)

​=> ​11​

​ ​print​(y)

​=> ​11​

As a side note, when you enable printing within a function, add an explanatory message to each printout:

​ ​print​(f​'This is x+1 in function add1: {x+1}'​)

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