Python Basics 3: File Handling - Input & Output

Files Input & Output

Files Built-in Functions:

• open() - 打开文件 open a file
• read() - 输入 read/input a file
• readline() - 输入一行 read/input a line
• seek() -文件内移动 look for
• write() -输出 output
• close() -关闭文件 close a file
• mode=w - 要写入 mode as write
• mode=r -要读取 mode as read

open() function

The open() function takes two parameters; filename, and mode.

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

• "r" - Read - Default value. Opens a file for reading, error if the file does not exist
• "a" - Append - Opens a file for appending, won’t overwrite any existing content, creates the file if it does not exist
• "w" - Write - Opens a file for writing, overwrites any existing content, creates the file if it does not exist
• "x" - Create - Creates the specified file, returns an error if the file exists
• "t" - Text - Default value. Text mode
• "b" - Binary - Binary mode (e.g. images)

Scenario 1: Create and open a file, write in and close

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# open a file and write
# the name.txt will be created
# open() -> write() -> close()

file1 = open('name.txt', 'w')

# write the info in the file
file1.write('Lily')
file1.close()

Scenario 2: Open the existing file and input the content

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# open the file and read 
# 注意这里没有mode'w' or 'r' or 'a'

file2 = open('name.txt')
print(file2.read())
file2.close()

>>>Lily

Scenario 3: Add new content in the existing file, but the rest content remains the same

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# Add another character in the file
# mode 'a'

file3 = open('name.txt', 'a')
file3.write('Luzy')
file3.close()

>>>LilyLucy

Scenario 4: Open the existing file, print out all lines

Traditional solution:

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fd = open('name.txt')

# not elegant way
try:
    for line in fd:
        print(line)
finally:
    fd.close()

More simplified solution:

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# more clean way - with
# no need to use finally
# if exception, with will be automatically calling finally to close the file

with open('name.txt') as f:
    for line in f:
        print(line)

File Operations

More specific scenarios of file handling:

Scenario 4: Read the first line

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file4 = open('name.txt')
print(file4.readline())

>>>Lily

Scenario 5: Read multiple lines

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file5 = open('name.txt')
for line in file5.readlines():
    print(line)
    print('=======')
    
>>>Lily
>>>=======
>>>Lucy
>>>=======

**Scenario 6: Look for where the file pointer is at **

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file6 = open('name.txt')
# check where the file pointer is at
print(file6.tell())

# move the file pointer to line 2
file.read(1)
print(file6.tell())

# move the file pointer to original line 1
file6.seek(0)
print(file6.tell())

>>>0
>>>1
>>>0

多个参数偏移

第一个参数代表偏移位置

第二个参数代表：

• 0 - 表示从文件开头偏移
• 1 - 表示从当前位置偏移
• 2 - 表示从文件结尾

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file6.seek(5,0)

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