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+演算子を使って文字列連結すると速度が落ちるということだったので  
以下のように、joinをひたすら繰り返すだけのプログラムを試しました

**test.py**

```

    test = ''
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    print(test)
    
```

すると、以下のようなエラーが出ます。

```

    Traceback (most recent call last):
      File "C:\Users\emoto\Desktop\test.py", line 10, in <module>
        test = test.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    MemoryError
    
```

実行環境は、Windows10の64bit、AnacondaPrompt、Pythonのバージョンは3.7です。  
+演算子の場合は以下のように文字列連結してもエラーになりません。

**test2.py**

```

    test = ""
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    test += "aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"+"aaaa"
    print(test)
    
```

また、以下のようにappendしてからjoinしてみるコードも試してみました。

**test3.py**

```

    test = []
    test.append(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test.append(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test.append(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    print(''.join(test))
    
```

すると、以下のようなエラーが出ました。

```

    Traceback (most recent call last):
      File "C:\Users\emoto\Desktop\test.py", line 5, in <module>
        print(''.join(test))
    TypeError: sequence item 0: expected str instance, list found
    
```

Pythonで複数の文字列を連結するにはどのように書くのがよいのでしょうか？  
この例では、"aaaa"という文字列だけを連結していますが  
実際の開発中のコードでは、途中でif文やfor文や変数等を組み合わせて文字列連結しています。
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2019-12-20T05:41:51.227
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2019-12-21T04:20:48.540
2019-12-20T05:56:51.660
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question
python
Pythonの文字列連結の方法について
`test3.py`のエラー原因は @metropolis
さんコメントで良いとして、`test.py`がエラーになって`test2.py`が正常に動作するのは、その2つの処理が同じ結果では無く`test.py`はそれだけ実際のメモリを使っているからです。  
だからテストの方法が悪いわけです。  
自分で確認するのは大切だとしても、何をやっているかは意識した方が良いでしょう。

* * *

`test2.py`では単純に指定された文字列が連結されていくだけですが、`test.py`ではその直前に連結した処理結果が指定された文字列の間に挟まります。  
例えば文字の数を減らして、行ごとに文字を変えれば一目瞭然でしょう。`test.py`ではアッという間にサイズが肥大します。  
`test.py`簡略化

```

    test = ''
    test = test.join(["a","a","a","a","a","a","a","a","a","a","a","a","a","a","a"])
    test = test.join(["b","b","b","b","b","b","b","b","b","b","b","b","b","b","b"])
    test = test.join(["c","c","c","c","c","c","c","c","c","c","c","c","c","c","c"])
    print(len(test))
    
```

これで結果の文字数が`3165`  
長いので"a"と"b"の連結だけの結果ではこうなります。

```

    baaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaabaaaaaaaaaaaaaaab
    
```

`test2.py`簡略化

```

    test = ""
    test += "a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"+"a"
    test += "b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"+"b"
    test += "c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"+"c"
    print(len(test))
    
```

こちらは結果の文字数が`45`  
同様に"a"と"b"の連結だけの結果ではこうなります。

```

    aaaaaaaaaaaaaaabbbbbbbbbbbbbbb
    
```

ちなみに`test.py`の元の長さで連結した場合は、8行分だけで`6811334100`=6.34G文字x2byte(Unicode)になります。  
9行目で`MemoryError`になりました。

* * *

`test.py`を`test2.py`と同様の条件にするとしたら、以下のようになるでしょう。  
各行毎に完了させる処理で代入となりますが、文字列の連結は`.join`だけで行われます。

```

    test = ''
    test = ''.join([test,"aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test = ''.join([test,"aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
      ...
    
```

* * *

例えばこの辺の記事を参考にしてみるとか。  
[文字列連結の常識、joinは本当に速いのか
Python3](https://qiita.com/GinRickey/items/d8ed9964820313340a93)  
[Pythonの処理速度を上げる方法
その１](https://aqvi.hatenadiary.org/entry/20080411/1207929090)  
[Pythonの処理速度を上げる方法
その２](https://aqvi.hatenadiary.org/entry/20080419/1208577161)  
[pythonの速度で気にするところ(高速化メモ)](http://nobunaga.hatenablog.jp/entry/2018/03/19/081425)
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2019-12-20T06:51:25.380
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2019-12-21T04:20:48.540
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answer
```

    j = ''
    test = ''
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    test += j.join(["aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa","aaaa"])
    print(test)
    
```
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2019-12-20T09:30:11.970
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2019-12-20T09:30:11.970
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answer