7 Scope of Variable

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From the course by Nanjing University

Data Processing Using Python

84 ratings

Nanjing University

Data Processing Using Python

84 ratings

This course (The English copy of "用Python玩转数据" <https://www.coursera.org/learn/hipython/home/welcome>) is mainly for non-computer majors. It starts with the basic syntax of Python, to how to acquire data in Python locally and from network, to how to present data, then to how to conduct basic and advanced statistic analysis and visualization of data, and finally to how to design a simple GUI to present and process data, advancing level by level. This course, as a whole, based on Finance data and through establishment of popular cases one after another, enables learners to more vividly feel the simplicity, elegance and robustness of Python. Also, it discusses the fast, convenient and efficient data processing capacity of Python in humanities and social sciences fields like literature, sociology and journalism and science and engineering fields like mathematics and biology, in addition to business fields. Similarly, it may also be flexibly applied into other fields. The course has been updated. Updates in the new version are : 1) the whole course has moved from Python 2.x to Python 3.x 2) Added manual webpage fetching and parsing. Web API is also added. 3) Improve the content order and enrich details of some content especially for some practice projects.

From the lesson

Basics of Python

Hi, guys, welcome to learn Module 01 “Basics of Python”! I’ll first guide you to have a glimpse of its simplicity for learning as well as elegance and robustness. Less is more: the author of Python must know this idea well. After learning this module, you can master the basic language structures, data types, basic operations, conditions, loops, functions and modules in Python. With them, we can write some useful programs!

1 Conditions10:12

4 break, continue and else in Loops11:37

5 Self-defined Functions14:21

6 Recursion7:30

7 Scope of Variable4:33

A1: Standard Library Functions14:21

A2: Exceptions10:11

Meet the Instructors

ZHANG Li
associate professor
Department of Computer Science

The scope of a variable

determines the visited scope of a variable.

Since no variable declaration is needed in Python,

the scope of a variable

is where it appears.

We usually do not pay much attention to the scope of a variable,

but it often causes great trouble to us if there is any problem.

So we can not ignore it.

As for a global variable or local variable,

we may consider it like this,

a variable in a function is a local variable,

while that in the body of program code is a global variable.

Let's look at a program,

global_str is

inside the body of program code.

Therefore, it is a global variable.

By contrast, local_str is located in the function foo().

Thus, it is a local variable.

During the execution of function,

local_str is visible within the function.

Sure, a global variable is also visible,

so the result of execution is "helloworld".

If there is such a case,

when a global variable and a local variable

share the same name, what will happen?

Like in this instance,

if we call the function f(),

will the result be 9 or 25?

It is 25.

The reason is, if a global variable and a local variable

share the same name, such a principle is observed --

the inner one will mask the outer one.

Think about another question.

If we want to modify

the value of a global variable,

can we do that in a function?

Let's look at this example.

There is a global variable "a", whose original value equals 3.

Sure, we may output its value inside the function.

Can this statement be properly executed?

It is possible to re-assign the global variable "a"

in some programming languages,

but an error occurs in Python, as we see.

The error says, the local variable "a"

referenced before assignment.

To assign a variable defined outside the function,

what shall we do?

This method seems obviously impossible.

In Python, you need to tell it,

this variable is not local, but global.

This requires the global statement.

Let's make a change to the program just now.

With "global a" plus this statement,

conduct assignment again.

The original value of a is 3, not changed to 5.

Let's see the program execution.

With f(8), the actual argument "8" is transferred to the formal argument "x".

"a" is a global variable emphasized with the global statement.

We output the value of "a", i.e. 3

and re-assign 5 to a.

For its attribute,

this statement can be properly executed.

Print the value of a+x, i.e. "5+8"

So, the second value is 13.

f(8) is completely executed.

The last statement is "print(a)".

We may think about it,

is it 3 or 5 that is printed at last?

Since "a" is a global variable emphasized by the global statement,

its value has been changed,

so the result is 5.

The final output results are 3, 13 and 5.

In programming,

we often require strong cohesion in modules,

and weak coupling with other modules.

Obviously, a global variable does not conform to this principle.

Certainly, we do not mean we should totally abandon global variables.

If the use of a global variable

can significantly improve

the performance of a program or make designing much easier,

we may appropriately

use some global variables.

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