class: big, middle

# Engineering 1020: Introduction to Programming

.title[
  .lecture[Lecture 21:]
  .title[Files and pickles]
  
  
]

.footer[[/lecture/21/](/lecture/21/)]

---

# Recall: what computers are

### Central processing unit

* executes _instructions_

???

We've seen how we can write **source code** (in Python or other languages),
which is translated by an **interpreter** (or a **compiler**) into
**instructions** that the CPU can execute.

### Memory

* a.k.a., _random access memory_
--

* stores information in _bits_

???

We've also seen some data types that a computer uses to store information in memory
(though we still need to talk about how those types are turned into 0s and 1s
 in memory).

### Input/output devices

???

The only part of these three components that we haven't spent much time on
is I/O: input and output.

---

# Files

### Persistent ordered sequences of bytes

### Organized in _directories_ (or _folders_)

---

# Files

### Persistent ordered sequences of bytes

### Organized in _directories_ (or _folders_)

---

# Accessing files

### We can _open_ files with `open()`:

```python
Help on built-in function open in module io:

open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)
    Open file and return a stream.  Raise OSError upon failure.
    
    [...]
    
    mode is an optional string that specifies the mode in which the file
    is opened. It defaults to 'r' which means open for reading in text
    mode.  Other common values are 'w' for writing (truncating the file if
    it already exists), 'x' for creating and writing to a new file, and
    'a' for appending (which on some Unix systems, means that all writes
    append to the end of the file regardless of the current seek position).
    [...]
```

---

# Reading and writing

### To read a file's contents as text:

```python
f = open("path/to/the/file")
contents = f.read()
```

### Or as raw bytes:

```python
f = open("path/to/the/file", mode="rb")
contents = f.read()
```

---

# Demo

.centered[
<img src="https://y.yarn.co/b40fbf38-ed97-4dfe-8fe0-e1878096df91_text.gif"
     width="800"
     />
]

---

# Writing text

### Use the `write()` method:

```python
f = open("path/to/the/file.txt", mode="w")
f.write("Hello, world!")
```

---

# Storing data

### We often want to _save_ data

.floatright[
<img src="https://www.tvguide.com/a/img/hub/2020/03/26/a7502ce9-a932-4071-b06b-bab08a64c66d/20326-startrekpicarddata-news.jpg"
     width="400"
     />
]

* longer-term storage (beyond one program execution)
--

* user data
--

* configuration
--

* history and statistics

---

# Serialization

### Several kinds of _serialization_ available:

* [JSON](https://docs.python.org/3/library/json.html#module-json)
* [`pickle`](https://docs.python.org/3/library/pickle.html)
* [Protobuf](https://protobuf.dev/getting-started/pythontutorial)
* and [many, many others](https://en.wikipedia.org/wiki/Comparison_of_data-serialization_formats)

---

# Summary

### Files: persistent arrays of bytes

### Directories / folders

### Reading and writing

### Serialization

---
class: big, middle

(here endeth the lesson)