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Practical Python Programming (course by @dabeaz)

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[Contents](../Contents.md) \| [Previous (3.4 Modules)](04_Modules.md) \| [Next (3.6 Design Discussion)](06_Design_discussion.md)
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# 3.5 Main Module
This section introduces the concept of a main program or main module.
### Main Functions
In many programming languages, there is a concept of a *main* function or method.
```c
// c / c++
int main(int argc, char *argv[]) {
...
}
```
```java
// java
class myprog {
public static void main(String args[]) {
...
}
}
```
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This is the first function that executes when an application is launched.
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### Python Main Module
Python has no *main* function or method. Instead, there is a *main*
module. The *main module* is the source file that runs first.
```bash
bash % python3 prog.py
...
```
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Whatever file you give to the interpreter at startup becomes *main*. It doesn't matter the name.
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### `__main__` check
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It is standard practice for modules that run as a main script to use this convention:
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```python
# prog.py
...
if __name__ == '__main__':
# Running as the main program ...
statements
...
```
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Statements enclosed inside the `if` statement become the *main* program.
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### Main programs vs. library imports
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Any Python file can either run as main or as a library import:
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```bash
bash % python3 prog.py # Running as main
```
```python
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import prog # Running as library import
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```
In both cases, `__name__` is the name of the module. However, it will only be set to `__main__` if
running as main.
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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.
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```python
if __name__ == '__main__':
# Does not execute if loaded with import ...
```
### Program Template
Here is a common program template for writing a Python program:
```python
# prog.py
# Import statements (libraries)
import modules
# Functions
def spam():
...
def blah():
...
# Main function
def main():
...
if __name__ == '__main__':
main()
```
### Command Line Tools
Python is often used for command-line tools
```bash
bash % python3 report.py portfolio.csv prices.csv
```
It means that the scripts are executed from the shell /
terminal. Common use cases are for automation, background tasks, etc.
### Command Line Args
The command line is a list of text strings.
```bash
bash % python3 report.py portfolio.csv prices.csv
```
This list of text strings is found in `sys.argv`.
```python
# In the previous bash command
sys.argv # ['report.py, 'portfolio.csv', 'prices.csv']
```
Here is a simple example of processing the arguments:
```python
import sys
if len(sys.argv) != 3:
raise SystemExit(f'Usage: {sys.argv[0]} ' 'portfile pricefile')
portfile = sys.argv[1]
pricefile = sys.argv[2]
...
```
### Standard I/O
Standard Input / Output (or stdio) are files that work the same as normal files.
```python
sys.stdout
sys.stderr
sys.stdin
```
By default, print is directed to `sys.stdout`. Input is read from
`sys.stdin`. Tracebacks and errors are directed to `sys.stderr`.
Be aware that *stdio* could be connected to terminals, files, pipes, etc.
```bash
bash % python3 prog.py > results.txt
# or
bash % cmd1 | python3 prog.py | cmd2
```
### Environment Variables
Environment variables are set in the shell.
```bash
bash % setenv NAME dave
bash % setenv RSH ssh
bash % python3 prog.py
```
`os.environ` is a dictionary that contains these values.
```python
import os
name = os.environ['NAME'] # 'dave'
```
Changes are reflected in any subprocesses later launched by the program.
### Program Exit
Program exit is handled through exceptions.
```python
raise SystemExit
raise SystemExit(exitcode)
raise SystemExit('Informative message')
```
An alternative.
```python
import sys
sys.exit(exitcode)
```
A non-zero exit code indicates an error.
### The `#!` line
On Unix, the `#!` line can launch a script as Python.
Add the following to the first line of your script file.
```python
#!/usr/bin/env python3
# prog.py
...
```
It requires the executable permission.
```bash
bash % chmod +x prog.py
# Then you can execute
bash % prog.py
... output ...
```
*Note: The Python Launcher on Windows also looks for the `#!` line to indicate language version.*
### Script Template
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Finally, here is a common code template for Python programs that run
as command-line scripts:
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```python
#!/usr/bin/env python3
# prog.py
# Import statements (libraries)
import modules
# Functions
def spam():
...
def blah():
...
# Main function
def main(argv):
# Parse command line args, environment, etc.
...
if __name__ == '__main__':
import sys
main(sys.argv)
```
## Exercises
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### Exercise 3.15: `main()` functions
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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
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should be able to run it interactively like this:
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```python
>>> import report
>>> report.main(['report.py', 'Data/portfolio.csv', 'Data/prices.csv'])
Name Shares Price Change
---------- ---------- ---------- ----------
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AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
GE 95 13.48 -26.89
MSFT 50 20.89 -44.21
IBM 100 106.28 35.84
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>>>
```
Modify the `pcost.py` file so that it has a similar `main()` function:
```python
>>> import pcost
>>> pcost.main(['pcost.py', 'Data/portfolio.csv'])
Total cost: 44671.15
>>>
```
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### Exercise 3.16: Making Scripts
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Modify the `report.py` and `pcost.py` programs so that they can
execute as a script on the command line:
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```bash
bash $ python3 report.py Data/portfolio.csv Data/prices.csv
Name Shares Price Change
---------- ---------- ---------- ----------
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AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
GE 95 13.48 -26.89
MSFT 50 20.89 -44.21
IBM 100 106.28 35.84
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bash $ python3 pcost.py Data/portfolio.csv
Total cost: 44671.15
```
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fix sample output in 03/05 exercises 3.15+16
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[Contents](../Contents.md) \| [Previous (3.4 Modules)](04_Modules.md) \| [Next (3.6 Design Discussion)](06_Design_discussion.md)
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