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David Beazley
2020-05-28 10:22:32 -05:00
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Notes/03_Program_organization/05_Main_module.md
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[Contents](../Contents) \| [Previous (3.4 Modules)](04_Modules) \| [Next (3.6 Design Discussion)](06_Design_discussion)
# 3.5 Main Module
This section introduces the concept of a main program or main module.
@@ -22,7 +24,7 @@ class myprog {
}
```
This is the first function that is being executing when an application is launched.
This is the first function that executes when an application is launched.
### Python Main Module
@@ -34,11 +36,11 @@ bash % python3 prog.py
...
```
Whatever module you give to the interpreter at startup becomes *main*. It doesn't matter the name.
Whatever file you give to the interpreter at startup becomes *main*. It doesn't matter the name.
### `__main__` check
It is standard practice for modules that can run as a main script to use this convention:
It is standard practice for modules that run as a main script to use this convention:
```python
# prog.py
@@ -53,22 +55,22 @@ Statements inclosed inside the `if` statement become the *main* program.
### Main programs vs. library imports
Any file can either run as main or as a library import:
Any Python file can either run as main or as a library import:
```bash
bash % python3 prog.py # Running as main
```
```python
import prog
import prog # Running as library import
```
In both cases, `__name__` is the name of the module. However, it will only be set to `__main__` if
running as main.
As a general rule, you don't want statements that are part of the main
program to execute on a library import. So, it's common to have an `if-`check in code
that might be used either way.
Usually, you don't want statements that are part of the main program
to execute on a library import. So, it's common to have an `if-`check
in code that might be used either way.
```python
if __name__ == '__main__':
@@ -221,7 +223,8 @@ bash % prog.py
### Script Template
Here is a common code template for Python programs that run as command-line scripts:
Finally, here is a common code template for Python programs that run
as command-line scripts:
```python
#!/usr/bin/env python3
@@ -251,8 +254,9 @@ if __name__ == '__main__':
### Exercise 3.15: `main()` functions
In the file `report.py` add a `main()` function that accepts a list of command line options and produces the same output as before.
You should be able to run it interatively like this:
In the file `report.py` add a `main()` function that accepts a list of
command line options and produces the same output as before. You
should be able to run it interatively like this:
```python
>>> import report
@@ -280,7 +284,8 @@ Total cost: 44671.15
### Exercise 3.16: Making Scripts
Modify the `report.py` and `pcost.py` programs so that they can execute as a script on the command line:
Modify the `report.py` and `pcost.py` programs so that they can
execute as a script on the command line:
```bash
bash $ python3 report.py Data/portfolio.csv Data/prices.csv