Master Python File I/O, Dialogs, and Exception Handling in One Guide

1. Open a File

Use the built‑in open(filename, mode) function, where filename is the path and mode specifies how the file is opened (e.g., "r" for reading, "w" for writing).

<code>input = open(r"/home/usr/test.txt", "r")
</code>

2. Write Data

2.1 File Object

Calling open returns a _io.TextIOWrapper instance that provides methods such as write() , read() , and close() .

2.2 Detect File Existence

Before writing, check whether a file already exists to avoid accidental overwrites:

<code>import os.path
if os.path.isfile("presidents.txt"):
    print("文件已经存在")
</code>

2.3 Read All Data

Use read() or readlines() for small files; for large files, iterate line by line:

<code>for line in infield:
    # process each line
</code>

Example that copies the contents of one file to another while counting lines and characters:

<code>import os.path, sys

def main():
    f1 = input("请输入原文件:").strip()
    f2 = input("请输入目标文件：").strip()
    if os.path.isfile(f2):
        print(f2 + "已经存在了")
        sys.exit()
    infile = open(f1, "r")
    outfile = open(f2, "w")
    countLines = countChars = 0
    for line in infile:
        countLines += 1
        countChars += len(line)
        outfile.write(line)
    print(countLines, "lines and", countChars, "chars copied")
    infile.close()
    outfile.close()

main()
</code>

2.4 Write Numbers

Convert numbers to strings before writing them with write() .

3. File Dialogs

The tkinter.filedialog module provides askopenfilename() and asksaveasfilename() for opening and saving files through a GUI dialog.

<code>from tkinter.filedialog import askopenfilename, asksaveasfilename

filenameforReading = askopenfilename()
print("you can read from" + filenameforReading)

filenameforWriting = asksaveasfilename()
print("you can write data to " + filenameforWriting)
</code>

4. Fetch Data from a Website

Use urllib.request.urlopen() to retrieve data from a URL, then decode the bytes to a string.

<code>import urllib.request

def main():
    url = input("输入网址: ").strip()  # e.g., https://www.baidu.com
    infile = urllib.request.urlopen(url)
    s = infile.read().decode()
    counts = countLetters(s.lower())
    for i in range(len(counts)):
        if counts[i] != 0:
            print(chr(ord('a') + i) + "出现" + str(counts[i]) + (" time" if counts[i] == 1 else " times"))

def countLetters(s):
    counts = [0] * 26
    for ch in s:
        if ch.isalpha():
            counts[ord(ch) - ord('a')] += 1
    return counts

main()
</code>

5. Exception Handling

Use try/except blocks to catch errors and keep the program running.

<code>try:
    # code that may raise an exception
except ExceptionType as ex:
    print("Exception:", ex)
</code>

Multiple except clauses can handle different exception types, optionally followed by else and finally blocks.

6. Using the Exception Object

<code>try:
    # risky code
except SomeError as ex:
    print("Exception:", ex)
</code>

7. Raising Exceptions

Raise an exception with raise ExceptionClass("message") . Example:

<code>raise RuntimeError("错误请求")
</code>

8. Custom Exception Classes

Define your own exception by inheriting from BaseException (or a subclass).

<code>class InvalidRadiusException(RuntimeError):
    def __init__(self, radius):
        super().__init__()
        self.radius = radius

class Circle:
    def __init__(self, radius):
        self.setRadius(radius)
    def getRadius(self):
        return self.__radius
    def setRadius(self, radius):
        if radius >= 0:
            self.__radius = radius
        else:
            raise InvalidRadiusException(radius)

def main():
    try:
        c1 = Circle(-5)
    except InvalidRadiusException as ex:
        print("半径是", ex.radius, "不合法")
    except Exception:
        print("一些异常")
    else:
        print("半径是", c1.getRadius())

main()
</code>

9. Binary I/O with Pickle

Use the pickle module to serialize (dump) and deserialize (load) Python objects to/from binary files.

<code>import pickle

# Write an object
with open('data.pkl', 'wb') as f:
    pickle.dump(my_object, f)

# Read objects until EOFError
with open('data.pkl', 'rb') as f:
    while True:
        try:
            obj = pickle.load(f)
            # process obj
        except EOFError:
            break
</code>