Category: Python

Remove a character from a string using Python

This post introduces how to remove / replace a character from a string using Python.

In Python, strings are immutable, so we need to create a new string. If you want to remove the ‘;’ wherever it appears. See below for an example.

Remove character”;”

>>> original_str = "good; ok"
>>> new_str = original_str.replace(";","")
>>> print(new_str)
good ok
>>>

See the examples below for different ways to check whether a string contains a specific character.

>>> s = "good; ok"
>>> ";" in s
True
>>> ";" not in s
False
>>> s.find(";") == -1
False
>>> s. find(";") != -1
True
>>> chars = set(";o")
>>> if any((c in chars) for c in s):
>>> print("Found")
Found
>>> import re
>>> pattern = re.compile(r";o")
>>> if pattern.findall(s):
>>> print("Found")
Found
>>>

 

References:

How to delete a character from a string using python?

Remove all special characters, punctuation and spaces from string

How to check a string for specific characters? (pdf)

Access PostgreSQL from Python using pscopg2

psycopg2 is a python module for PostgreSQL.

  • Basic module usage

http://initd.org/psycopg/docs/usage.html#basic-module-usage

  • with statement

Starting from version 2.5, psycopg2’s connections and cursors are context managers and can be used with the with statement:

http://initd.org/psycopg/docs/usage.html#with-statement

  • Server side cursors

http://initd.org/psycopg/docs/usage.html#server-side-cursors

When a database query is executed, the Psycopg cursor usually fetches all the records returned by the backend, transferring them to the client process. If the query returned an huge amount of data, a proportionally large amount of memory will be allocated by the client.

If the dataset is too large to be practically handled on the client side, it is possible to create a server side cursor. Using this kind of cursor it is possible to transfer to the client only a controlled amount of data, so that a large dataset can be examined without keeping it entirely in memory.

 

Posts Referenced:

 

Ways to use double quotes inside a string variable in Python

To use double quotes inside a string variable in Python, see below for the three ways:

1) Use single and double quotes together:

>>> print '"words to be double-quoted"'
"words to be double-quoted"

2) Escape the double quotes within the string:

>>> print "\"words to be double-quoted\""
"words to be double-quoted"

3) Use triple-quoted strings:

>>> print """ "words to be double-quoted" """
"words to be double-quoted"

Note: when you need to use single quote within a single quote, the above methods work for single quote case as well.

References:

Create a hash table for large data in python

This post introduces how to create a hash table in python.

Text files can have duplicates which will overwrite existing keys in your dictionary (the python name for a hash table). We can create a unique set of the keys, and then use a dictionary comprehension to populate the dictionary.

sample_file.txt:

a
b
c
c

 

Python code:

with open("sample_file.txt") as f:
  keys = set(line.strip() for line in f.readlines())
my_dict = {key: 1 for key in keys if key}

>>> my_dict
{'a': 1, 'b': 1, 'c': 1}