Pythonでは関数のデフォルト値は最初の呼び出ししか評価されない。

func-arg-default.ipynb

{
 "cells": [
  {
   "cell_type": "code",
   "execution_count": 1,
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   "source": [
    "def hoge(arg=[]):\n",
    "    arg.append(\"hoge\")\n",
    "    return arg"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 2,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['hoge']\n"
     ]
    }
   ],
   "source": [
    "h = hoge()\n",
    "print(h)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['hoge', 'hoge']\n"
     ]
    }
   ],
   "source": [
    "h = hoge()\n",
    "print(h)"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 4,
   "metadata": {
    "collapsed": false
   },
   "outputs": [
    {
     "name": "stdout",
     "output_type": "stream",
     "text": [
      "['hoge', 'hoge', 'hoge']\n"
     ]
    }
   ],
   "source": [
    "h = hoge()\n",
    "print(h)"
   ]
  }
 ],
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