改訂２０１１／11/13

	Function
	Functions are named sequences of statements that execute some task.

We have already used functions, such as:

>>> type('GAATTC')
<type 'str'>
>>> len(cds)
852
      

For example len is a function that calculates the length of things and we asked here for the length of our DNA sequence cds.

	Function call
	Function calls are statements that execute or call a function. The Python syntax of function calls is the function name followed by a comma separated list of arguments inclosed into parentheses. Even if a function does not take any argument the parentheses are necessary.

Differences between function calls and variables.  As variable names, function names are stored in a namespace with a reference to their corresponding sequence of statements. When they are called, their name is searched in the namespace and the reference to their sequence of statements is returned. The procedure is the same as for variable names. But unlike them, the following parentheses indicate that the returned value is a sequence of statements that has to be executed. That's why they are even necessary for functions which are called without arguments.

	Arguments of functions
	Arguments are values provided to a function when the function is called. We will see more about them soon.

.1. Defining Functions

In Section 3.5 we have learnt how to apply or call functions. So let's remember the example calculating the GC-percentage of a DNA sequence.

>>> float(count(cds, 'G') + count(cds, 'C')) / len(cds)
      

This calculates the gc percentage of the specific DNA sequence cds, but we can use the same formula to calculate the gc percentage of other DNA sequences. The only thing to do is to replace cds by the new DNA sequence in the formula. But it is not very convenient to remember the formula and retype it all the time. It would be much easier to type the following instead.

>>> gc('ATGCAT')
0.33333333333333331
>>> gc(cds)
0.54460093896713613
      

The only thing we have to remember is the name of the new function and its use.

	Abstraction
	The possibility to define such new function executing tasks specified by yourself, is an abstraction feature, provided by all high level programming languages.

	Important
	It is also important to notice that functions have to be defined before they are called. You can not use something that is not defined.

Here is the syntax of such a new definition in Python:

>>> from string import *

>>> def gc(seq):
...     return float(count(seq, 'G') + count(seq, 'C')) / len(seq)
      

Let's have a closer look at this definition. Figure 8.1 illustrates the structure of a function definition.

Figure 8.1. Function definitions

def and return are basic instructions.

	Basic instruction
	Basic instructions are statements that define the language rules and the semantic of Python. They constitute the basic set of instructions of Python. Each basic instruction has its own syntax that you have to learn in order to master the programming language.

The return basic instruction is used to return the result of a function back, in our example the value of the GC percentage of the specified DNA sequence.

The def basic instruction indicates to Python that a function definition follows. It has to be followed by the new function name and a comma separated list of parameter names enclosed into parentheses.

	Parameter
	Parameters are variable names. When the function is called they are bound in the same order to the arguments given.

The body of a function contains the piece of code needed to execute the subtask of the function. In the example above, the body contains only the return statement. Here is a more complex example that excludes ambiguous bases from the GC percentage calculation.

>>> from string import *

>>> def gc(seq):
...    nbases = count(seq, 'N')
...    gcpercent = float(count(seq, 'G') + count(seq, 'C')) / (len(seq) - nbases)
...    return gcpercent
      

In this example the body of the function contains three instructions (2 assignments and the return statement). The body of a function follows the definition line and is written as an indented block initiated by a colon.

	Block
	Blocks are structure elements of a program, that are used to group instructions. They are initiated by a colon and are separated from the rest of the code by the same indentation step in Python.

In our example, the function body is written as a block to separate it from the rest of the program. The Python interpreter used it to detect where the function body starts and ends.

Important

In other programming languages special words or characters are used to indicate the begin and the end of blocks and the indentation is only used to make the code readable for humans. In Python the indentation is used explicitly to do this. So in the above example of a function definition the end of the body is detected when the next line starts without any indentation step. This is also illustrated in Figure 8.2. Example 8.1. More complex function definition >>> from string import * >>> def gc(seq): ... nbases = count(seq, 'N') ... gcpercent = float(count(seq, 'G') + count(seq, 'C')) / \ (len(seq) - nbases) ... return gcpercent ... >>> gc('ATGCN') 0.5

Figure 8.2. Blocks and indentation

---

7.7. Solutions		8.2. Parameters and Arguments or the difference between a function definition and a function call

以上http://www.pasteur.fr/formation/infobio/python/より

１関数とは
 にはあらかじめ用意されている組込関数と自分で作る関数
 と２種類ある。利用の方法は全く同じである.

　名前（引数）の形式で書かれる
　ｌｅｎ（ｘ）　　　　引数Ｘについて何かをする　ｌｅｎは組み込み関数
　>>> s=[1,2,3,4,5,6]
　>>> len(s)
　6
　
 関数のメリットは
 １．繰り返し作業をコード化する
 ２．処理の内容が短縮される
 ３．処理の変更が容易になる。

ホーム　　　　　戻る