Python - Reading & Writing Files

# Reading & Writing Files

Week 10

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## Review: Modules

### What is a Python Module?

Another Python file.

### How do You Import a Module?

```py
import module_name
import module_name as module_alias
from module_name import something
from module_name import something as something_alias
```

---

## Review: Functions

### How Do You Call a Function?

```py
print()
print("Hello, World!")
print("Hello, World!", end="")
```

### How Do You Define a Function?

```py
def add_numbers(num1, num2):
  result = num1 + num2
  return result
```

### Is This Valid Python?

```py
print("Calling pring")
print_variable = print
print_variable("Calling print_variable")
```

Yes!

NOTES:
- `print_variable` stores a reference to `print` function
- `print_variable` is now callable!

### Is This Valid Python?

```py
say_hello()
def say_hello():
  print("Hello, World!")
```

No!

---

## Review: Files

### Name the 3 Steps to Read a File

1. Open 
2. Read 
3. Close

### Name the 3 Steps to Write a File

1. Open 
2. Write 
3. Close

### How to Auto-close a File?

```py
with open('notes.txt', 'r') as file:
  content = file.read()
```

### What Are File "Modes?"

Tells the OS how you will use the file.

```py
file = open('notes.txt', 'r')
print( file.read() )
file.close()
```

### What Kinds of File Modes Are There?

```py
'rt' # read text (same as 'r')
'rb' # read binary
'wt' # write text (same as 'w')
'wb' # write binary
'at' # append text (same as 'a')
'ab' # append binary
```

### What Happens When You Open a File That Doesn't Exist?

In read mode (`'r'`) Python throws an error.

In write/append mode (`'w'`/`'a'`) the file is created.

### How to Avoid File DNE Error?

```py
import os
if os.path.exists("my_file.txt"):
  # TODO use file
```

NOTE: `try`/`catch` is another way

---

## Tabular Data

### What is Tabular Data?

> Refers to data that is organized in a table with rows and columns <sup>[<a href="https://www.statology.org/tabular-data/">1</a>]</sup>

### Why is it Useful?

- Organization 
- Data analysis 
- Scalability

### Where Have You Seen It?

- Google Sheets 
- Microsoft Excel

### Databases

- Databases are special programs that store _massive_ amounts of data
- Databases _structure data_ just like tabular data
- Databases do not store all the data in a single file

NOTE:
This is how data is stored for large-scale, professional apps.
They often allow data to be _related_ for performance and analysis.

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## Pandas Module

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## Exercise