Want to restrict Date to current system date

Date Restriction

I want to restrict the date field, like t=only the current system date should be available to be selected.
So what function i need to add and where? pls help me with the solution.


The following is the library_transaction.py filecode of mine
from __future__ import unicode_literals
import frappe
from frappe import _
from frappe.model.document import Document

class LibraryTransaction(Document):
    def validate(self):
        last_transaction = frappe.get_list("Library Transaction",
            fields=["transaction_type", "transaction_date"],
            filters = {
                "article": self.article,
                "transaction_date": ("<=", self.transaction_date),
                "name": ("!=", self.name)
            })
        if self.transaction_type=="Issue":
            msg = _("Article {0} {1} has not been recorded as returned since {2}")
            if last_transaction and last_transaction[0].transaction_type=="Issue":
                frappe.throw(msg.format(self.article, self.article_name,
                    last_transaction[0].transaction_date))
        else:
            if not last_transaction or last_transaction[0].transaction_type!="Issue":
                frappe.throw(_("Cannot return article not issued"))
                ------------------------------------
                
  now library_transaction.js of mine is 
  
  
  frappe.ui.form.on("Library Transaction", "library_membership",
    function(frm) {
        frappe.call({
            "method": "frappe.client.get",
            args: {
                doctype: "Library Membership",
                name: frm.doc.library_membership
            },
            callback: function (data) {
                frappe.model.set_value(frm.doctype,
                    frm.docname, "member_name", 
                    data.message.member_first_name
                    + (data.message.member_last_name ?
                        (" " + data.message.member_last_name) : ""))
				frappe.model.set_value(frm.doctype,
                    frm.docname, "is_paid_member", 
                    ((data.message.paid == 0) ? "No" : "Yes"))
            }
        })
    });
    ===================================
    
   @Nabin as You Said- "You should write the following code in validate:"
   and so i did but still not able to restrict to make only current system date enable(Selectable). May be i am palcing it at a wrong place. Please correct me if so...
    
    from __future__ import unicode_literals
import frappe
from frappe import _
from frappe.model.document import Document
from frappe.utils import nowdate

class LibraryTransaction(Document):
    def validate(self):
        last_transaction = frappe.get_list("Library Transaction",
            fields=["transaction_type", "transaction_date"],
            filters = {
                "article": self.article,
                "transaction_date": ("<=", self.transaction_date),
                "name": ("!=", self.name)
            })
        if self.transaction_type=="Issue":
            msg = _("Article {0} {1} has not been recorded as returned since {2}")
            if last_transaction and last_transaction[0].transaction_type=="Issue":
                frappe.throw(msg.format(self.article, self.article_name,
                    last_transaction[0].transaction_date))
        else:
            if not last_transaction or last_transaction[0].transaction_type!="Issue":
                frappe.throw(_("Cannot return article not issued"))
		if self.transaction_date != nowdate():
			frappe.throw(_("Transaction can only be made at current date"))