Day 16 Frequently-used modules
一 Regular Expression
A RegEx, or Regular Expression, is a sequence of characters that forms a search pattern.
RegEx can be used to check if a string contains the specified search pattern.
Metacharacters元字符
Metacharacters are characters with a special meaning
| Character | Description | Example |
|---|---|---|
| [] | A set of characters | "[a-m]" |
| \ | Signals a special sequence (can also be used to escape special characters) | "\d" |
| . | Any character (except newline character) | "he..o" |
| ^ | Starts with | "^hello" |
| $ | Ends with | "world$" |
| * | Zero or more occurrences | "aix*" |
| + | One or more occurrences | "aix+" |
| {} | Exactly the specified number of occurrences | "al{2}" |
| | | Either or | "falls|stays" |
| () | Capture and group |
Special Sequences
A special sequence is a \ followed by one of the characters in the list below, and has a special meaning:
| Character | Description | Example |
|---|---|---|
| \A | Returns a match if the specified characters are at the beginning of the string | "\AThe" |
| \b | Returns a match where the specified characters are at the beginning or at the end of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string")C | r"\bain" r"ain\b" |
| \B | Returns a match where the specified characters are present, but NOT at the beginning (or at the end) of a word (the "r" in the beginning is making sure that the string is being treated as a "raw string") | r"\Bain" r"ain\B" |
| \d | Returns a match where the string contains digits (numbers from 0-9) | "\d" |
| \D | Returns a match where the string DOES NOT contain digits | "\D" |
| \s | Returns a match where the string contains a white space character | "\s" |
| \S | Returns a match where the string DOES NOT contain a white space character | "\S" |
| \w | Returns a match where the string contains any word characters (characters from a to Z, digits from 0-9, and the underscore _ character) | "\w" |
| \W | Returns a match where the string DOES NOT contain any word characters | "\W" |
| \Z | Returns a match if the specified characters are at the end of the string | "Spain\Z" |
Sets
A set is a set of characters inside a pair of square brackets [] with a special meaning:
| Set | Description |
|---|---|
| [arn] | Returns a match where one of the specified characters (a, r, or n) are present |
| [a-n] | Returns a match for any lower case character, alphabetically between a and n |
| [^arn] | Returns a match for any character EXCEPT a, r, and n |
| [0123] | Returns a match where any of the specified digits (0, 1, 2, or 3) are present |
| [0-9] | Returns a match for any digit between 0 and 9 |
| [0-5][0=9] | Returns a match for any two-digit numbers from 00 and 59 |
| [a-zA-Z] | Returns a match for any character alphabetically between a and z, lower case OR upper case |
| [+] | In sets, +, *, ., |, (), $,{} has no special meaning, so [+] means: return a match for any + character in the string |
二 RE in python
Python has a built-in package called re, which can be used to work with Regular Expressions.
When you have imported the re module, you can start using regular expressions:
| Function | Description |
|---|---|
| findall | Returns a list containing all matches |
| search | Returns a Match object if there is a match anywhere in the string |
| split | Returns a list where the string has been split at each match |
| sub | Replaces one or many matches with a string |
The findall() Function
The findall() function returns a list containing all matches.
Example Print a list of all matches:
import re
txt = "The rain in Spain"
x = re.findall("ai", txt)
print(x)
The list contains the matches in the order they are found.
If no matches are found, an empty list is returned:
import re
txt = "The rain in Spain"
x = re.findall("Portugal", txt)
print(x)
The search() Function
The search() function searches the string for a match, and returns a Match object if there is a match.
If there is more than one match, only the first occurrence of the match will be returned:
Example Search for the first white-space character in the string:
import re
txt = "The rain in Spain"
x = re.search("\s", txt)
print("The first white-space character is located in position:", x.start())
If no matches are found, the value None is returned:
import re
txt = "The rain in Spain"
x = re.search("Portugal", txt)
print(x)
The split() Function
The split() function returns a list where the string has been split at each match:
import re
txt = "The rain in Spain"
x = re.split("\s", txt, 1)
print(x)
The sub() Function
The sub() function replaces the matches with the text of your choice:
import re
txt = "The rain in Spain"
x = re.sub("\s", "9", txt,2)
print(x)
三 Python JSON module
JSON is a syntax for storing and exchanging data.
JSON is text, written with JavaScript object notation.
Parse /pɑːz/解析 JSON
Convert from JSON to Python
If you have a JSON string, you can parse it by using the json.loads() method.
The result will be a Python dictionary.
Convert from Python to JSON
If you have a Python object, you can convert it into a JSON string by using the json.dumps() method.
| Python | JSON |
|---|---|
| dict | Object |
| list | Array |
| tuple | Array |
| str | String |
| int | Number |
| float | Number |
| True | true |
| False | false |
| None | null |
四 Python time Module
Python time.time()
The time() function returns the number of seconds passed since epoch/ˈiːpɒk/新纪元 .
For Unix system, January 1, 1970, 00:00:00 at UTC is epoch (the point where time begins).
import time
seconds = time.time()
print("Seconds since epoch =", seconds)
Python time.sleep()
The sleep() function suspends (delays) execution of the current thread for the given number of seconds.
import time
print("This is printed immediately.")
time.sleep(2.4)
print("This is printed after 2.4 seconds.")
Python time.localtime()
The localtime() function takes the number of seconds passed since epoch as an argument and returns struct_time in local time.
import time
result = time.localtime(1545925769)
print("result:", result)
print("\nyear:", result.tm_year)
print("tm_hour:", result.tm_hour)
When you run the program, the output will be something like:
result: time.struct_time(tm_year=2018, tm_mon=12, tm_mday=27, tm_hour=15, tm_min=49, tm_sec=29, tm_wday=3, tm_yday=361, tm_isdst=0)
year: 2018
tm_hour: 15
Python time.gmtime()
The gmtime() function takes the number of seconds passed since epoch as an argument and returns struct_time in UTC.
import time
result = time.gmtime(1545925769)
print("result:", result)
print("\nyear:", result.tm_year)
print("tm_hour:", result.tm_hour)
When you run the program, the output will be:
result = time.struct_time(tm_year=2018, tm_mon=12, tm_mday=28, tm_hour=8, tm_min=44, tm_sec=4, tm_wday=4, tm_yday=362, tm_isdst=0)
year = 2018
tm_hour = 8
