Python内置函数示例

Guide to Python Built-in Functions
Mathematical Functions
abs() – returns the absolute value of a number
pow() – raises a number to a specified power
round() – rounds a number to a specified number of decimal places
max() – returns the largest value in a list or tuple
min() – returns the smallest value in a list or tuple

print(abs(-10)) # Output: 10
print(pow(2, 3)) # Output: 8
print(round(3.14159, 2)) # Output: 3.14
print(max(2, 5, 8, 1)) # Output: 8
print(min(2, 5, 8, 1)) # Output: 1

divmod() - returns the quotient and remainder when one number is divided by another
sum() - returns the sum of all the elements in a list or tuple
round() - rounds a number to the nearest integer
floor() - returns the largest integer less than or equal to a given number
ceil() - returns the smallest integer greater than or equal to a given number

print(divmod(10, 3)) # Output: (3, 1)
my_list = [1, 2, 3, 4, 5]
print(sum(my_list)) # Output: 15
print(round(3.7)) # Output: 4

import math
print(math.floor(3.9)) # Output: 3
print(math.ceil(3.1)) # Output: 4

String Functions
len() – returns the length of a string
capitalize() – capitalizes the first letter of a string
replace() – replaces a specified substring with another substring
split() – splits a string into a list of substrings based on a specified delimiter
join() – joins a list of strings into a single string using a specified delimiter

text = "hello world"
print(len(text)) # Output: 11
print(text.capitalize()) # Output: Hello world
print(text.replace("world", "Python")) # Output: hello Python
print(text.split(" ")) # Output: ['hello', 'world']
words = ["hello", "world"]
print(" ".join(words)) # Output: hello world

lower() – returns a string in all lowercase letters
upper() – returns a string in all uppercase letters
strip() – removes leading and trailing whitespace from a string
startswith() – returns True if a string starts with a specified substring
endswith() – returns True if a string ends with a specified substring

text = " Hello, World! "
print(text.lower()) # Output: " hello, world! "
print(text.upper()) # Output: " HELLO, WORLD! "
print(text.strip()) # Output: "Hello, World!"
print(text.startswith("Hello")) # Output: False
print(text.endswith("!")) # Output: True

List Functions
append() – adds an element to the end of a list
remove() – removes the first occurrence of a specified element from a list
sort() – sorts the elements of a list
reverse() – reverses the order of the elements in a list
count() – returns the number of times a specified element appears in a list

fruits = ["apple", "banana", "cherry"]
fruits.append("orange")
print(fruits) # Output: ['apple', 'banana', 'cherry', 'orange']
fruits.remove("banana")
print(fruits) # Output: ['apple', 'cherry', 'orange']
fruits.sort()
print(fruits) # Output: ['apple', 'cherry', 'orange']
fruits.reverse()
print(fruits) # Output: ['orange', 'cherry', 'apple']
print(fruits.count("cherry")) # Output: 1

extend() – adds the elements of one list to the end of another list
index() – returns the index of the first occurrence of a specified element in a list
insert() – inserts an element at a specified index in a list
pop() – removes and returns the element at a specified index in a list
clear() – removes all the elements from a list

my_list = [1, 2, 3]
my_list.extend([4, 5])
print(my_list) # Output: [1, 2, 3, 4, 5]
print(my_list.index(3)) # Output: 2
my_list.insert(1, 6)
print(my_list) # Output: [1, 6, 2, 3, 4, 5]
removed_element = my_list.pop(2)
print(my_list) # Output: [1, 6, 3, 4, 5]
print(removed_element) # Output: 2
my_list.clear()
print(my_list) # Output: []

Dictionary Functions
keys() – returns a list of all the keys in a dictionary
values() – returns a list of all the values in a dictionary
items() – returns a list of all the key-value pairs in a dictionary
get() – returns the value associated with a specified key
pop() – removes the key-value pair associated with a specified key

person = {"name": "John", "age": 25, "country": "USA"}
print(person.keys()) # Output: dict_keys(['name', 'age', 'country'])
print(person.values()) # Output: dict_values(['John', 25, 'USA'])
print(person.items()) # Output: dict_items([('name', 'John'), ('age', 25), ('country', 'USA')])
print(person.get("name")) # Output: John
person.pop("age")
print(person) # Output: {'name': 'John', 'country': 'USA'}

update() - updates the keys and values in a dictionary with another dictionary or key-value pairs
clear() - removes all the key-value pairs from a dictionary
copy() - returns a shallow copy of a dictionary
popitem() - removes and returns a random key-value pair from a dictionary

person = {"name": "John", "age": 25, "country": "USA"}
person2 = {"name": "Jane", "city": "New York"}
person.update(person2)
print(person) # Output: {'name': 'Jane', 'age': 25, 'country': 'USA', 'city': 'New York'}
person.clear()
print(person) # Output: {}
person = person2.copy()
print(person) # Output: {'name': 'Jane', 'city': 'New York'}
person.popitem()
print(person) # Output: {'name': 'Jane'}

File Handling Functions
open() – opens a file and returns a file object
read() – reads the contents of a file
write() – writes data to a file
close() – closes a file object

# Open a file
file = open("example.txt", "w")

# Write data to the file
file.write("Hello, world!")

# Close the file
file.close()

# Open the file again and read its contents
file = open("example.txt", "r")
print(file.read()) # Output: Hello, world!
file.close()

File Handling Functions (continued)
readline() – reads a single line from a file
readlines() – reads all the lines from a file and returns them as a list
writelines() – writes a list of strings to a file
seek() – sets the current position in a file
tell() – returns the current position in a file

# Open a file
file = open("example.txt", "r")

# Read a single line from the file
line = file.readline()
print(line) # Output: Hello, world!

# Read all the lines from the file and return them as a list
lines = file.readlines()
print(lines) # Output: ['Hello, world!\\\\n', 'This is a test file.\\\\n']

# Close the file
file.close()

# Open the file again and write a list of strings to it
file = open("example.txt", "w")
file.writelines(["This is line 1.\\\\n", "This is line 2.\\\\n", "This is line 3.\\\\n"])
file.close()

# Open the file again and read its contents
file = open("example.txt", "r")
print(file.read()) # Output: This is line 1.\\\\nThis is line 2.\\\\nThis is line 3.\\\\n
file.close()

# Open the file again and set the current position to the beginning of the file
file = open("example.txt", "r")
file.seek(0)

# Read the first two characters from the file
print(file.read(2)) # Output: Th

# Get the current position in the file
print(file.tell()) # Output: 2

# Close the file
file.close()

Type Conversion Functions
int() – converts a string or float to an integer
float() – converts a string or integer to a float
str() – converts any data type to a string
list() – converts any iterable to a list

print(int("42")) # Output: 42
print(float("3.14")) # Output: 3.14
print(str(42)) # Output: "42"
print(list("hello")) # Output: ['h', 'e', 'l', 'l', 'o']

bool() – converts a value to a Boolean (True or False) value
tuple() – converts any iterable to a tuple
set() – converts any iterable to a set
dict() – converts a sequence of key-value pairs to a dictionary

print(bool(0)) # Output: False
print(bool(1)) # Output: True
print(tuple("hello")) # Output: ('h', 'e', 'l', 'l', 'o')
print(set([1, 2, 3, 2, 1])) # Output: {1, 2, 3}
print(dict([("name", "John"), ("age", 25), ("country", "USA")])) # Output: {'name': 'John', 'age': 25, 'country': 'USA'}

Input and Output Functions
input() – gets input from the user
print() – prints output to the console

name = input("What is your name? ")
print("Hello, " + name + "!")

OS Module
The OS module comes under Python’s standard utility modules.
The *os* and *os.path* modules include many functions to interact with the file system.

Handling the Current Working Directory

 Getting the Current working directory
To get the location of the current working directory os.getcwd() is used.

import os 
cwd = os.getcwd() 
print("Current working directory:", cwd) 

Output:
Current working directory: /home/nikhil/Desktop/gfg

 Changing the Current working directory
To change the current working directory(CWD) os.chdir() method is used.

import os 
def current_path(): 
    print("Current working directory before") 
    print(os.getcwd()) 
    print() 
current_path() 
os.chdir('../') 
current_path()

Output:
Current working directory before
C:\Users\Nikhil Aggarwal\Desktop\gfg
Current working directory after
C:\Users\Nikhil Aggarwal\Desktop

Creating a Directory

 Using os.mkdir()

import os
directory = "GeeksforGeeks"
parent_dir = "D:/Pycharm projects/"
path = os.path.join(parent_dir, directory)
os.mkdir(path)
print("Directory '% s' created" % directory)
directory = "Geeks"
parent_dir = "D:/Pycharm projects"
mode = 0o666
path = os.path.join(parent_dir, directory)
os.mkdir(path, mode)
print("Directory '% s' created" % directory)

Output:

Directory 'GeeksforGeeks' created
Directory 'Geeks' created
 Using os.makedirs()
os.makedirs() method in Python is used to create a directory recursively.
That means while making leaf directory if any intermediate-level directory is missing, os.makedirs() method will create them all.

Example: This code creates two directories, “Nikhil” and “c”, within different parent directories.
It uses the os.makedirs function to ensure that parent directories are created if they don’t exist.

It also sets the permissions for the “c” directory.
The code prints messages to confirm the creation of these directories

import os
directory = "Nikhil"
parent_dir = "D:/Pycharm projects/GeeksForGeeks/Authors"
path = os.path.join(parent_dir, directory)
os.makedirs(path)
print("Directory '% s' created" % directory)
directory = "c"
parent_dir = "D:/Pycharm projects/GeeksforGeeks/a/b"
mode = 0o666
path = os.path.join(parent_dir, directory)
os.makedirs(path, mode)
print("Directory '% s' created" % directory)

Output:

Directory 'Nikhil' created
Directory 'c' created
Listing out Files and Directories with Python
There is os.listdir() method in Python is used to get the list of all files and directories in the specified directory.
If we don’t specify any directory, then the list of files and directories in the current working directory will be returned.

Example: This code lists all the files and directories in the root directory (“/”).
It uses the os.listdir function to get the list of files and directories in the specified path and then prints the results.

import os 
path = "/"
dir_list = os.listdir(path) 
print("Files and directories in '", path, "' :") 
print(dir_list) 

Output:

Files and directories in ' / ' :
['sys', 'run', 'tmp', 'boot', 'mnt', 'dev', 'proc', 'var', 'bin', 'lib64', 'usr', 
'lib', 'srv', 'home', 'etc', 'opt', 'sbin', 'media']
Deleting Directory or Files using Python
OS module proves different methods for removing directories and files in Python.
These are – 

Using os.remove()
Using os.rmdir()

 Using os.remove() Method
os.remove() method in Python is used to remove or delete a file path.
This method can not remove or delete a directory.
If the specified path is a directory then OSError will be raised by the method.

Example: Suppose the file contained in the folder are:



This code removes a file named “file1.txt” from the specified location “D:/Pycharm projects/GeeksforGeeks/Authors/Nikhil/”.
It uses the os.remove function to delete the file at the specified path.

import os 
file = 'file1.txt'
location = "D:/Pycharm projects/GeeksforGeeks/Authors/Nikhil/"
path = os.path.join(location, file) 
os.remove(path) 

Output:



 Using os.rmdir()
os.rmdir() method in Python is used to remove or delete an empty directory.
OSError will be raised if the specified path is not an empty directory.

Example: Suppose the directories are 



This code attempts to remove a directory named “Geeks” located at “D:/Pycharm projects/”.

It uses the os.rmdir function to delete the directory.
If the directory is empty, it will be removed.
If it contains files or subdirectories, you may encounter an error.

import os 
directory = "Geeks"
parent = "D:/Pycharm projects/"
path = os.path.join(parent, directory) 
os.rmdir(path)

Output:



Commonly Used Functions
 Using os.name function
This function gives the name of the operating system dependent module imported.
The following names have currently been registered: ‘posix’, ‘nt’, ‘os2’, ‘ce’, ‘java’ and ‘riscos’.

import os
print(os.name)

Output:

posix
Note: It may give different output on different interpreters, such as ‘posix’ when you run the code here.

 Using os.error Function
All functions in this module raise OSError in the case of invalid or inaccessible file names and paths, or other arguments that have the correct type, but are not accepted by the operating system.
os.error is an alias for built-in OSError exception.

This code reads the contents of a file named ‘GFG.txt’.
It uses a ‘try…except‘ block to handle potential errors, particularly the ‘IOError‘ that may occur if there’s a problem reading the file.

If an error occurs, it will print a message saying, “Problem reading: GFG.txt.” 

import os
try:
    filename = 'GFG.txt'
    f = open(filename, 'rU')
    text = f.read()
    f.close()
except IOError:
  print('Problem reading: ' + filename)

Output: 

Problem reading: GFG.txt
 Using os.popen() Function
This method opens a pipe to or from command.
The return value can be read or written depending on whether the mode is ‘r’ or ‘w’.
Syntax: 

 os.popen(command[, mode[, bufsize]])
Parameters mode & bufsize are not necessary parameters, if not provided, default ‘r’ is taken for mode.

This code opens a file named ‘GFG.txt’ in write mode, writes “Hello” to it, and then reads and prints its contents.
The use of os.popen is not recommended, and standard file operations are used for these tasks.

import os
fd = "GFG.txt"
file = open(fd, 'w')
file.write("Hello")
file.close()
file = open(fd, 'r')
text = file.read()
print(text)
file = os.popen(fd, 'w')
file.write("Hello")

Output: 

Hello
Note: Output for popen() will not be shown, there would be direct changes into the file.

 Using os.close() Function
Close file descriptor fd.
A file opened using open(), can be closed by close()only.
But file opened through os.popen(), can be closed with close() or os.close().
If we try closing a file opened with open(), using os.close(), Python would throw TypeError.

import os
fd = "GFG.txt"
file = open(fd, 'r')
text = file.read()
print(text)
os.close(file)

Output: 

Traceback (most recent call last):
  File "C:\Users\GFG\Desktop\GeeksForGeeksOSFile.py", line 6, in 
    os.close(file)
TypeError: an integer is required (got type _io.TextIOWrapper)
Note: The same error may not be thrown, due to the non-existent file or permission privilege.

 Using os.rename() Function
A file old.txt can be renamed to new.txt, using the function os.rename().
The name of the file changes only if, the file exists and the user has sufficient privilege permission to change the file.

import os
fd = "GFG.txt"
os.rename(fd,'New.txt')
os.rename(fd,'New.txt')

Output:

Traceback (most recent call last):
  File "C:\Users\GFG\Desktop\ModuleOS\GeeksForGeeksOSFile.py", line 3, in 
    os.rename(fd,'New.txt')
FileNotFoundError: [WinError 2] The system cannot find the
file specified: 'GFG.txt' -> 'New.txt'
 A file name “GFG.txt” exists, thus when os.rename() is used the first time, the file gets renamed.

Upon calling the function os.rename() second time, file “New.txt” exists and not “GFG.txt”  thus Python throws FileNotFoundError.


 Using os.remove() Function
Using the Os module we can remove a file in our system using the os.remove() method.
To remove a file we need to pass the name of the file as a parameter.

import os #importing os module.
os.remove("file_name.txt") #removing the file.

The OS module provides us a layer of abstraction between us and the operating system.

When we are working with os module always specify the absolute path depending upon the operating system the code can run on any os but we need to change the path exactly.
If you try to remove a file that does not exist you will get FileNotFoundError.

 Using os.path.exists() Function
This method will check whether a file exists or not by passing the name of the file as a parameter.
OS module has a sub-module named PATH by using which we can perform many more functions.

import os 
#importing os module
result = os.path.exists("file_name") #giving the name of the file as a parameter.
print(result)

Output:

False
As in the above code, the file does not exist it will give output False.
If the file exists it will give us output True.

 Using os.path.getsize() Function
In os.path.getsize() function, python will give us the size of the file in bytes.
To use this method we need to pass the name of the file as a parameter.

import os #importing os module
size = os.path.getsize("filename")
print("Size of the file is", size," bytes.")

Output:

Size of the file is 192 bytes.