Python处理CSV文件数据

class: center, middle, inverse, title-slide

# Python处理CSV文件数据
## 🌱
### 吴燕丰
### 江西财大，金融学院
### 2020/10/29

---

### CSV格式数据

CSV: Comma-Seperated Values (逗号分隔值)

[财经大学列表.csv](财经大学列表.csv)

```
江西财经大学,江西,南昌
上海财经大学,上海,杨浦
西南财经大学,四川,成都
东北财经大学,辽宁,大连
```

也可以添加标题行，比如：

```
学校名称,所在省市,城市或区
江西财经大学,江西,南昌
上海财经大学,上海,杨浦
西南财经大学,四川,成都
东北财经大学,辽宁,大连
```

---

### CSV module（模块）

读操作 `csv.reader()`

```python
import csv

with open('财经大学列表.csv', mode='r',
          newline='', encoding='utf-8') as csvfile:
    csvreader = csv.reader(csvfile)
    for row in csvreader:
        print(f'{row[0]}位于 {row[1]} 省/市 {row[2]} 市/区')
```

```
## 江西财经大学位于 江西 省/市 南昌 市/区
## 上海财经大学位于 上海 省/市 杨浦 市/区
## 西南财经大学位于 四川 省/市 成都 市/区
## 东北财经大学位于 辽宁 省/市 大连 市/区
```

.footnote[
**注意**：`newline=''`，`encoding='utf-8'`，`mode='r'`
]

---

### CSV module（模块）

写操作 `csv.writer()`

```python
import csv

with open('aNewFile.csv', mode='w', newline='', 
          encoding='utf-8') as f:
    writer = csv.writer(f)
    writer.writerow(['吴燕丰','男','江西'])
    writer.writerow(['Maxwell','Male','Scotland'])
    writer.writerow(['Turing','Male','England'])
    writer.writerow(['Donald Knuth','Male','America'])
    writer.writerow(['Marie Curie','Female','Poland'])
```

```
## 10
## 23
## 21
## 27
## 27
```

.footnote[
提示：写一行用`writerow()`，写多行用`writerows()`，`mode='a'`表示追加写入，不覆盖已有内容
]

---

### 文件读写

可参考网站：

- [Python File(文件) 方法](https://www.runoob.com/python/file-methods.html)

- [读写文件（Python官方文档）](https://docs.python.org/zh-cn/3/tutorial/inputoutput.html#reading-and-writing-files)

例子：

```python
f = open('filename.txt', mode='r', encoding='utf-8')
content = f.read()
```

---

### DataFrame 与 CSV

DataFrame是模块[pandas](https://pandas.pydata.org/pandas-docs/stable/user_guide/10min.html)的一个数据结构，需先安装模块pandas，然后使用它。
在Anaconda Prompt窗口输入

```bash
pip install pandas -i https://pypi.tuna.tsinghua.edu.cn/simple
```

- pandas 读 csv 文件

[pandas.read_csv()](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.read_csv.html#pandas.read_csv)

```python
import pandas as pd

df = pd.read_csv('aNewFile.csv', header=None)
df.head()
```

```
##               0       1         2
## 0           吴燕丰       男        江西
## 1       Maxwell    Male  Scotland
## 2        Turing    Male   England
## 3  Donald Knuth    Male   America
## 4   Marie Curie  Female    Poland
```

---

### DataFrame 与 CSV（续）

- pandas 写 csv 文件

[pandas.DataFrame.to_csv](https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.to_csv.html#pandas.DataFrame.to_csv)

```python
import pandas as pd

df = pd.DataFrame({
 '姓名': ['吴燕丰','Maxwell','Turing','Donald Knuth','Marie Curie'],
 '性别': ['男','Male','Male','Male','Female'],
 '籍贯': ['江西','Scotland','England','America','Poland']
})
df.to_csv('saved_from_DataFrame.csv')
```

---
### Python File Open

**open()**

There are four different methods (modes) for opening a file:

|mode|meaning|
|----|-------|
|"r"| Read - Default value. Opens a file for reading, error if the file does not exist|
|"a"| Append - Opens a file for appending, creates the file if it does not exist|
|"w"| Write - Opens a file for writing, creates the file if it does not exist|
|"x"| Create - Creates the specified file, returns an error if the file exists|

In addition you can specify if the file should be handled as binary or text mode

|mode|meaning|
|----|-------|
|"t"| Text - Default value. Text mode|
|"b"| Binary - Binary mode (e.g. images)|

---
### Open a file in a different location

If the file is located in a different location, you will have to specify the file path, like this:

### &#127792;

Open a file on a different location:

```python
f = open("D:\\myfiles\welcome.txt", "r")
print(f.read())
```

---
### Read Only Parts of the File

### &#127792;

Return the 5 first characters of the file:

```python
f = open("demofile.txt", "r")
print(f.read(5))
```

### &#127792;

Read two lines of the file:

```python
f = open("demofile.txt", "r")
print(f.readline())
print(f.readline())
```

---
### Write to an Existing File

To write to an existing file, you must add a parameter to the **open()** function:

- "a" - Append - will append to the end of the file
- "w" - Write - will overwrite any existing content

**append**

```python
f = open("demofile2.txt", "a")
f.write("Now the file has more content!")
f.close()
f = open("demofile2.txt", "r")
print(f.read())
```

**overwrite**

```python
f = open("demofile3.txt", "w")
f.write("Woops! I have deleted the content!")
f.close()
f = open("demofile3.txt", "r")
print(f.read())
```

---
### Create a New File

To create a new file in Python, use the **open()** method, with one of the following parameters:

- "x" - Create - will create a file, returns an error if the file exist

- "a" - Append - will create a file if the specified file does not exist

- "w" - Write - will create a file if the specified file does not exist

Example

Create a file called "myfile.txt":

```python
f = open("myfile.txt", "x")
```

Create a new file if it does not exist:

```python
f = open("myfile.txt", "w")
```

---
### Python Delete File

Remove the file "demofile.txt":

```python
import os
os.remove("demofile.txt")
```

### Check if File exist

Check if file exists, then delete it:

```python
import os
if os.path.exists("demofile.txt"):
  os.remove("demofile.txt")
else:
  print("The file does not exist")
```

---
### Delete Folder

Remove the folder "myfolder":

```python
import os
os.rmdir("myfolder")
```